Argue for or against Manifest Destiny.
During this period, Americans were brought to believe that their westward expansion, including the taking of lands belonging to Mexico and Canada was divinely ordained.
The idea of Manifest Destiny was misguided and wrong, but understandable.
During the mid-1800s, it is not surprising that Americans thought that they had a God-given right to expand. To them, it was clear that their way of life was superior to that of the Native Americans and of the Spanish in Mexico. The US was more prosperous and more technologically advanced than either of those groups and this naturally would have led them to feel superior.
However, this does not make the idea right. Manifest Destiny was essentially based on the idea that worldly success was a sign of superiority. The Americans believed that their superior technology and more modern economy meant that they were morally superior to other groups. There is simply no way that this can be true.
To take an extreme example, the slaveowners of the South were technologically superior to, and more modern than, the slaves they took from Africa. But were they morally superior? The fact that they kept slaves makes it obvious that they were not.
Worldly power and success does not prove moral superiority. Therefore, the idea of Manifest Destiny was severely flawed.