Starting a war with Mexico would be a pretext for interning everyone of "Mexican" ethnicity, citizen or otherwise, as was done to Japanese nationals.

Its mincing words a bit, but an attack targeting drug cartel assets wouldn't necessarily be viewed as a war with Mexico. It could lead to that for sure, and the Mexican government could declare it an act of war, but we did just see the US literally invade a foreign country and arrest their sitting leader without war being declared on either side.