We are not in a state of cease-fire armistice with Mexico, as South Korea is with North Korea. If Congress wishes to declare war on Mexico, then sure, lay a minefield, but not before.
No we’re in a state of undeclared war, sort of like Israel vs. Hamas, Hezbollah, and the PLO (aka PA). Same difference.
Actually, the Mexican Army has invaded the United States over 200 times between 1996—2005 (latest graphic I have from DHS).
Several of those times the Mexican Army engaged (i.e., shot at) Border Patrol Agents and local law enforcement. That is an act of war.
So, you may be correct we are not in a state of cease-fire, however, we are in a case of active engagement.
The Mexican Army is hired by the cartels to escort drug smugglers.