Was the United States justified in going to war with Mexico?

It is highly unlikely that the U.S. was justified; and
more likely that the U.S. provoked an unnecessary dispute with Mexico that devolved into
war.

Manifest Destiny was a prevailing issue in U.S.
history at the time of the war, in fact James K. Polk had based his presidential
campaign on America's manifest destiny to overspread the continent. At the time,
Americans had moved to California and Texas in large numbers. Texas had recently fought
its own war of independence from Mexico and secured its freedom under the treaty of San
Jacinto which set the Mexican-Texas border at the Neuces River. Later, when the U.S.
annexed Texas as a state, the border was set at the Rio Grande. An additional
complication occurred when Mexico claimed the Treaty of San Jacinto was invalid as it
had been signed by Santa Anna under duress. Also, President Polk offered to buy
California from Mexico, a proposal which did not appeal to
Mexico.

Mexico was angered by the U.S. annexation of Texas
and broke diplomatic relations. In the meantime, President Polk sent a message to
General Thomas O. Larkin in Monterrey, California that he would not try to induce
California to join the union, but he further stated:

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if the people should desire to unite their
destiny with ours, they would be received as
brethren.

Polk then sent Gen.
Zachary Taylor to "guard" the Rio Grande, even though its designation as the legal
border of Texas was disputed. Several American troops were killed in a skirmish with
Mexican troops following which Polk delivered a war message to Congress, claiming that
"American blood has been shed on American soil." This, of course was a matter of some
debate; in fact Abraham Lincoln, then a Whig Congressman from Illinois, introduced
"spot" resolutions, asking Polk to designate the spot on American soil where American
blood had been shed.

The U.S. then declared war on Mexico,
which was ill prepared. By the resultant treaty of Guadalupe Hidalgo, Mexico
relinquished its claims to Texas, and the U.S. gained California and parts of New Mexico
and Arizona. The U.S. in turn paid Mexico $15 million and assumed all American claims
against Mexico. There was no significant advantage gained by the war that could not have
been obtained by peaceful means; therefore the war was hardly
justified.