How would Transcendentalists view the Mexican-American War? What would they think the causes and goals were? Would they think it's worth the effort?
Seeing as Henry David Thoreau spent a night in jail over his refusal to pay poll taxes which he refused on the grounds that they would support the war effort, it is pretty safe to say that they in general were against it.
Thoreau wrote a short treatise: On Civil Disobedience that details this issue. I suggest you read it! It also has significance in having major influence on the non-violent protests of MLK and Ghandi.