Was the annexation of the territory owned by Mexico, history homework help

Here is a very interesting web site done by PBS on the U.S.–Mexican American War.

http://www.pbs.org/kera/usmexicanwar/index_flash.html (Links to an external site.)

Was the ‘annexation’ of the territory owned by Mexico and gained through war a just act by the United States? Additionally, was there a link between expansionism and slavery as embodied in the Wilmot Proviso?

About 2 to 3 paragraphs is good