What spurred american trade with northern mexico?

The answer is the revolution that declared Mexico as an independent country. Both countries knew that they can benefit from it in terms of free trade and migration which promotes economic development. Spain was not happy of such negotiation. They believe that the trade will jeopardize the safety of Mexico. Mexico paid no heed and continued with the agreement with the United States.

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.

thanos obviously donald trump aint got nothin on thanos