I agree. If we were to invade Canada unprovoked, and to indiscriminately lay waste to the country, it would be perfectly understandable if Mexico suddenly decided to arm up and join an alliance that they thought could protect them. In fact, in such a situation it would be foolish to just sit there hoping that you don’t become the next target.
If we were to invade Mexico stating that their narco state is a clear and present danger to the USA, set up a 100 mile wide border buffer zone to intercept Illegal Colonizers... do you suppose Canada would take offense at that & start arming up?