Funny, I thought we fought that war with the British. Canada was part of Britain at that time, or did you forget? Why did we fight that war with the British? It was because they weren't respecting the sovereignty of our citizens. They were pretending that our citizens were THEIR citizens, sort of like pretending our land was still their land, but that may be too much of a complication for you to follow it.
We also fought a war with Mexico in 1848, and came very close to another in 1916, but they were not organized to hold up their end.
Mostly a land grab. Not sure there is any other way to slice it than to say that the U.S. sent troops into Mexico to convince them to let go of their ownership of Texas.
Not a good example to bring up for your position.
Texas was independent, and had been for about 10 year when the US annexed Texas. The 1848 war had to do with Mexican attacks on Americans in the area between the Rio Grand and the Nueces. Of course the Slave Power had some interest in getting more land south of the southern border of Missouri, and when Texas was annexed, they had the option to break the state into as many as 5 states, so to be used to balance the increasing number of Free states.
Another reason why the pretended southern concern with northern states getting outnumbered was a falsehood. They had the means for up to 4 more states to hand in Texas.