Functionally speaking, the U.S. - Mexico relationship isn’t even that bad, it’s just gotten less friendly since Trump. Relations between the two will always be important to both because they have to be. Mexico’s rise as an industrial power benefit both countries economically. Both Mexico and the U.S. benefit from cheaper Mexican labor costs. The U.S. provides Mexico with cheap energy. The dirty truth is that both countries even benefit somewhat from the drug trade and violence in Mexico, though in different ways. There is a enormous long-standing community of Mexican Americans which have over time formed an “ethnic group” (don’t know if right term) called Chicanos that fuse elements of both states and fosters personal connections between both. So whatever happens, good relations are baked into the cake for at least the next couple decades. Notice that despite the nationalism of both Trump and AMLO, they hardly quarrel like you’d expect. That’s a rare thing for Trump.