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 States ?
How similar is our current political situation in Irag and Afghanistan linked with this idea of manifest destiny?
Spain's influence in North America had been weakening for some time prior to the war; Mexico was therefore not able to be effective in keeping the ever-expanding US out of its territories, so war was inevitable. The Monroe Doctrine helped define the concept of Manifest Destiny, which was a polite way of covering the US desire to spread from coast to coast and pole to pole, regardless of the wishes other countries or cultures the ever-expanding population came across. Many of these areas did become physically part of the United States, and were able to do so only because there were enough settlers from the East who wished to create new states, and there were no other effective opposing forces, either from Europe or from the Native Americans. The US was successful in imposing its culture on those areas. However, the concept of Manifest Destiny does not apply to the Middle East -- these areas will never be contiguous to the United States, or settled by people from other states, and there is an effective opposition repelling US influence (the most effective being religious fanaticism, which is nearly impossible to counter, regardless if it be Christian or Muslim). Manifest Destiny may have been the argument for the United States to define its borders and become a nation, but overseas empire-building occurring now is a process and argument of a different nature.
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.