The nature of States and their relation to their citizens is very different in 2009 than it was in 1909, and even moreso than in 1839. If the game is to reflect historical plausibility, it should reflect the attitudes and aspirations of the people the states represent in the timeperiod the game is set in, not contemporary 21st century political attitudes.
If the United States, a functioning democracy with universal male suffrage for all citizens of European ancestry, can declare war on its neighbor, defeat it, annex 1/3 of that neighbors' land, and not only preserve its democratic institutions, but be able to further expand its democratic institutions afterward, then I see no reason why the player of the USA in Victoria should not be able to do the same.
as long as the attitudes of people (the political preferences) and the policy of the ruling party or overall coalition at that date allows for it, there is no problem.
however, the said escapade with mexica wasnt an unwarranted, 'hey i want to conquer' one either. there has been political questions and issues in regard to the status of settlers, after texas, and these were used as a casus belli.
similarly usa didnt decide to attack canada in the morning and in the afternoon its armies were marching towards the border at any point in its history. any act of hostility needed a justification to the houses and the people. regardless of the justification was right and sound, or fake and fabricated.
Even better, that said USA player should not be prevented from being able to seize the colonial possessions of a weaker competing colonial power because it is a liberal democracy, as that liberal democratic political system certainly did not prevent it historically from defeating Spain in 1898 and seizing control of Cuba, Puerto Rico and the Philippines.
spanish war wasnt a war that occurred because someone in american administration decided that spanish colonies were weak and 'ripe for taking', and then just outright happened. it passed through all the stages of build up, political tension, casus belli and execution as it happens for all wars for a democratic nation :
http://en.wikipedia.org/wiki/Spanish–American_War
but, what is more, actually the rule of houses (parliamentary system) even almost caused us to lose the war, had it not been for secretary of navy robertson pulling off a total fraud to build the navy up before the spanish war :
http://en.wikipedia.org/wiki/Amphitrite_class_monitor#The_Robeson_subterfuge
because congress cut and withheld funding for navy after the civil war, american navy was reeking.
even with the fraud the navy wasnt able to get new ships.
only in 1882 the subsequent congress decided to release funds for completion of the ships ordered back in 1870 :
By the time the Garfield administration assumed office in 1881, the Navy's condition had deteriorated still further. A review conducted on behalf of the new Secretary of the Navy, William H. Hunt, found that of 140 vessels on the Navy's active list, only 52 were in an operational state, of which a mere 17 were iron-hulled ships, including 14 ageing Civil War era ironclads. Hunt recognized the necessity of modernizing the Navy, and set up an informal advisory board to make recommendations.[14]
as you can see, in 19th century houses ruled democratic nations. not wannabee fascists.