What were America's motives for waging war in Mexico in 1848 ?Does the war against Mexico represent the values Americas associated with the United...

The motive that most historians give for our war with Mexico is the idea of Manifest Destiny/the desire to gain more land for the United States.

To some extent, the war did represent American values.  We wanted, in part, to spread our system of government over more of the continent and we saw Mexico as a backwards society whose governmental system was worse than ours. So the war is partly in line with the American value of spreading democracy.

On the other hand, waging war to get more land is not really in line with American values.

As far as Iraq and Afghanistan go, you could argue that they're related to Manifest Destiny (though that term is usually only applied to our taking of North America).  They are definitely related to the idea of our trying to spread our system of government and values to other countries.