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.
its a lot more expensive than the US properly controlling what weapons are leaving its borders.
rather than arming the cartels to fight against the mexican government, thr US could just... not
The US hasn't declared war since World War II.
I suspect Mexicans would view it as another Pancho Villa Expedition, which was also event where neither side declared war.
Yet. It has certainly ratcheted up worldwide tensions, to put it mildly.