As usual eurofed is indulging in his lust for massive empires. Of course so many things get in the way including that weakness of the early US, the hostility towards other races and religions that would be a lot harder to overcome than he assumes, especially if they make the WASP US a minority in its own country and the fact that most of those people would not want to be ruled by Washington anyway,
Being the enthusiastic advocate of successful empires sometimes is a
dirty job but it has its
satisfactions.
For the Terran Imperium!! May It Reach Beyond the Stars!! Nonetheless, a successful expansionist America is pretty much one of the most benevolent cases one can think of for playing this role, if one looks to its overall historical record.
Weakness of the early USA did not stop them from giving the oh-so-glorious British Empire a mighty bloody nose and a draw (admittably with all-important direct or indirect foreign assistance), an outcome the British never seriously tried to reverse. It seems far from outlandish to adjust the event sequence that led to this to ensure Canada too falls in the same net as the 13 colonies. Moreover, post-Napoleonic Spain was so weak it got its butt on a plate even from creole militias with no foreign assistance. It would only go worse in a conflict with the USA. And America defeated Mexico so thoroughly that choice of peace terms was entirely Washington's choice.
Nobody means to negate the impact of US bigotry as a countering force to Manifest Destiny (especially against Blacks), which is why I suggested various ways it could be lessened in the right circumstances. But we also should avoid the bias of exaggerating its might or making it an eternal, unchanging force. The influence of Anti-Catholicism in the USA often gets massively exaggerated: there are the signatures of Catholics on the Declaration of Independence and the Constitution, Canada was pre-approved for membership in the Articles of Confederation, Catholics enjoyed full civil and political rights and nobody really tried to make them second-class citizens. The terms of prejudice also shift over time and most minorities do enjoy increasing acceptance and social integration over time. Within a few generations, several of them (e.g. Catholics and European immigrant communities) were assimilated into the White majority so thoroughly the very notion of WASP has been dead and buried even among White supremacists for almost a century, others (e.g. Asians) developed an overwhelming reputation as "model minorities". There are good reasons to assume in the medium term, Latinos are on a track to repeat the same process. Evidence suggests Blacks are an especially unlucky special case that got the worst deal because of its circumstances of slavery and segregation, and even so, their status has massively changed in the last two centuries. There is no good reason to assume what worked for immigrants would not work pretty much the same way for inhabitants of annexed territories.
Moreover, acquisition of Canada, Northern Mexico, the Greater Antilles, and a couple bits of Central America would not get in the same city as 'making the WASP a minority in their own country' before integration makes the notion meaningless, and even the case of getting All of Mexico in the mid 19th century would at most make the Hispanics a 20%-25% minority within the USA. This proportion is unlikely to change so much as to upset the demographic balance over time since integration of Mexico within the USA would inevitably mean the region is going to experience demographic transition to the same pattern as the USA much earlier and more efficiently. Nonetheless, I myself regard US acquisition of Mexico as perhaps best being done gradually in a couple stages, the Northern portion in the early-mid 19th century, the rest a couple generations later, for reasons that have relatively little to do with exaggeration of WASP bigotry or romanticization of anti-American insurgency.
Last but not least, the historical record about US territorial acquisitions is overwhelmingly in favor of the locals coming to regard US rule as desirable or at least inevitable fairly quickly, with any initial armed resistance being thoroughly crushed. Notwithstanding the tiresome and insufferable ahistorical glorification and romanticization that guerrilla warfare got because of a few special cases such as Vietnam, the overwhelming historical evidence indicates guerrilla warfare tends to fare very poorly barring circumstances such as powerful external help and/or the enemy being so weak for its own reasons as to be on the verge of revolutionary collapse or so fettered by humanitarian political constraints its fighting effectiveness is all but abolished. The usual historical fate of insurgents w/o these perks was to get slaughtered. This totally applied in the 19th and early 20th century, and for the armed conflicts the USA fought in this period. The typical fate of hypothetical Canuck, Mexican, or Caribbean guerrillas fighting US rule would be to follow the footsteps of Sitting Bull, Geronimo, Sandino, and the Pilipino insurgents, not to be early equivalents of the Vietcong.
Cuba and the Dominican Republic came very close to being annexed in the 1870s and 1890s-1900s. Change a few votes in the Congress at the time, and they would be US states today, or at the very least (but less likely) the same as Puerto Rico. I won't deny the factors you quote had some importance, this is why I propose the occurrence of other variables that would nullify or counterbalance them. E.g. early acquisition of Canada and/or gradual transition of the Upper South states to voluntary abolition of slavery would so strengthen the free section it would hardly feel threatened get in the way of the slave section if it advocates some southward expansion to try and compensate. Chances are they would deem it a cheap way of keeping national concord. Likewise, Southern racism is hardly going to be a serious obstacle to expansion in the Caribbean if Reconstruction strangled Jim Crow in the cradle or sent most freedmen back to Africa, wiping out segregationism as a major political force.
I acknowledge myself that absorption of Mexico in the USA is probably best done in a couple stages. E.g. the northern portion (not much more populated than the almost empty OTL Mexican Cession) in 1848-49 and the heavily populated central and southern portion in 1890-1910. Having said that, I don’t see the difficulties you quote as unsurmountable as you make them. First, it is almost certain the USA *could* hold Mexico, barring something as extreme as the CSA winning the civil war. Any nationalist Mexican rebels would almost surely suffer the fate of Black Hawk, Sitting Bull, Geronimo, Sandino, and Macario Sakay. The pre-1945 US army was entirely able to put down all cases of guerrilla warfare armed resistance to US rule w/o any noticeable strain for the nation, including the Philippines, which were just as big, had a terrain even more difficult, and were more distant from the US core than Mexico.
As it concerns the deal of Mexicans within the USA, they are certainly going to get full religious equality, just as all the other US Catholics since the beginning of the nation. As it concerns racial equality, chances are the Creole elites are going to be socially assimilated in American ones and find their worthy place in the US political game w/o any real trouble, just as it happened in Louisiana. By the time the Mexican lands are set up in organized territories and later states, the assimilation process is going to be advanced enough, and the benefits of US rule in terms of political stability, economic opportunities, and representation so evident in comparison to the screwed-up state of independent Mexico, nobody in the mainstream is ever going to make a serious claim for independence. If anything, they are going to make claims to get statehood as soon as possible, and then shift to make claims about their states and section getting as many benefits as possible, same as the rest of the Union.
As it concerns the deal of the Amerindian lower classes, I do expect they are probably going to remain in their pre-annexation status of social and political subordination to the Creole elites for a good while. Then they are going to stage a gradual political movement for their socio-economic emancipation and break the hold of the Creole elites on power. It would be not so different from the Black civil rights movement, and end with ultimate success in a similar way, probably with less difficulty since prejudice against Amerindians in American society has never been so intense as the one against Blacks. The important point is the Hispanic Amerindian movement is surely going to have socio-political inclusion and civil rights as an objective, not independence. No disadvantaged minority in America has ever embraced nationalism and separatism as its main objective. The Amerindian lower classes in US Mexico are going to be no exception. Basically speaking, it is going to be the Chicano Movement on steroids, not Irish-style separatism.
As it concerns the status of slavery in the Mexican territories, it is obviously going to be an important political issue, and probably going to have a patchwork outcome. Broadly speaking, I expect slavery shall spread in the lightly populated areas that are closest to established slave states and have a terrain somewhat favorable to plantation economy shall adopt slavery, such as northeastern Mexico. It shall mostly fail in the heavily populated Mexican heartland because of the political resistance of the locals backed by the free section. The outcome may vary in northwestern Mexico, but I expect it shall be the same as in OTL Southwest.
This statement inexplicably dismisses the case of the USA getting Canada during the American Revolution. It was one of the best ways to do it, be it the case of British colonial authorities bungling the management of Quebecois and Maritime settlers with repressive policies and driving them to join the Patriots, or a better prepared and managed invasions of Quebec and Nova Scotia, or a mix of the above. All of that was eminently doable in the right circumstances. It also ignores the case of the USA absorbing Canada in a multi-step process. First, US diplomats get Upper Canada in the 1783 peace negotiations, as they almost did. This makes the area part of the US Midwest instead of a Loyalist haven and ensures Rupert’s Land and the Pacific Northwest are cut off to British settlers and wide open to American ones. In a few decades, Western Canada is inevitably going to fall into the US lap, be it the Texas way or the Louisiana way. The USA can then acquire Quebec and the Maritimes by siding against the British Empire in any equivalent of WWI.
Moreover, US victory in the war of 1812 and conquest of Canada (or at least acquisition of Upper and Western Canada as the first phase of northern expansion, see above) was entirely doable in 1812-14 if America prepares adequately for the war. Of course, it greatly help if the defeat of Napoleon is averted or at least significantly delayed. If the Americans win the land war and overrun settled Canada while Napoleon still stands unconquered and there is no victory in sight in Europe, chances are the British government is going to cut its losses and sign off the Canadian colonies, much like it did with the 13 colonies a few decades before. It is basically going to be treated as an extension of the outcome of the ARW.
Despite all the tiresome stereotypes about the “Never Surrender” indomitable and invincible British Empire, it did not fight the ARW to the bitter end for the sake of imperial pride or the Loyalists. In the right circumstances, defeat in the War of 1812 is going to look to the British just like defeat in the ARW, especially if it happens when Napoleon is still undefeated. And once the ink on the peace treaty is dry, Britain is never going to seek a rematch about Canada on its own initiative, just like it never seriously tried to reconquer the USA. Neither the Canadian colonies nor an handful of Loyalist Canucks were simply not that important to the British Empire in 1783 or 1812-14, especially since Britain demonstrably let the North American Loyalists be hung out to dry in the face of military defeat.
New England hostility to the conflict is going to be muted if the US army is victorious, since nothing changes people’s minds like success. I agree if the ARW and War of 1812 windows fail to be properly exploited, in most circumstances a good opportunity for the USA to conquer Canada won’t materialize until ca. 1870 (scrape 5-10 years or so if the ACW does not happen or it is not as long and destructive as OTL). Otherwise, most is not the same as all. E.g. if the 1830s liberal reforms in Britain are delayed up to the point of bringing the country into a revolutionary situation, and the 1837 Canadian rebellions happen at the same time, a timely US intervention would allow America to annex Canada with excessive trouble. The same would apply if Britain is busy fighting a major uprising in Ireland or India, or a war in Europe, when America makes its move.
US conquest of Australia and New Zealand was entirely doable by WWI. At that time, the USA had all the industrial and economic capability to outbuild the RN in the Atlantic or the Pacific if it put its mind to the task, and to harness the necessary logistics for an invasion of Australasia. If it gets done when the USA already controls all of North America, and Britain is busy fighting a major war in Europe, it becomes quite easy, as World Wars scenarios go. Any US intervention on the side of the CP in WWI is going to end with the Kaiser and the US President dictating terms of surrender to the British Parliament in a London ravaged by food shortages, economic collapse, and revolution. The HSF and the USN joining hands is a death sentence for the RN, once American industry goes into war mode.
A defeat of this kind would spell the end of the British Empire and wholly discredit its cause. You can bet the Indians are totally going for the door, even in the unlikely case the victors do not actively drive them that way. Inhabitants of the conquered Dominions are going to accept their new destiny of US citizens in relatively short order, since it would appear inevitable, collaboration with the new order offers a very good deal, cultural affinities with the Americans are overwhelming, and romantic loyalty to a defeated and fallen empire is not very appealing.
You may notice I mentioned US assimilation of S America as the possible end–stage culmination of a two-century process that gradually led the USA to absorb all of North America and Australasia during the 18th, 19th, and early 20th century. The success in this kind of process, with the massive benefits it would bring to America, would indeed represent a good justification for a radical change in the worldview of American public opinion in the favor of activist Pan-Americanism. Successful incorporation of Hispanic North America in the USA with all the vast benefits in terms of stable democracy, first-world standards of living, and elimination of the many problems plaguing Latin America since independence may also provide a compelling argument for many South Americans to follow the same path, screw nationalism. Of course, it does not need, and likely would not, happen all at once, or even for the whole continent.
One possible early step in this direction might be a North American-Australasian internationalist USA deciding to intervene in Colombia and/or Venezuela to overthrow a Maduro-style anti-American dictatorship that created a humanitarian disaster, or to wipe out the drug cartels and insurgent organizations in bed with them. Once US intervention turns the situation to the better, policymakers and public opinion in the USA and the involved areas come to embrace the notion that gradual assimilation of northern S America in the USA is the best long-term solution. After all, this version of the USA already has a 15-20 Hispanic states which are seamlessly integrated in the Union, everybody but an anti-American ideologue can agree living standards in them are much better than in independent S America, and northern S America has plenty of valuable resources. US Latinos have been assimilated in the White majority or acknowledged as a “model minority” for generations; American multiculturalism is a plain fact. Most objections to the proposal concern the economic costs of assimilating the proposed new states to American standards and fears a large land border with Brazil and Peru might lead to uncontrolled illegal immigration, which has been historically limited thanks to the short and well-controlled Panama border.