This response is not surprising. From the first moments of his candidacy, Trump railed against Mexico. Even before that, he complained about Mexico on Twitter, in part because he won a lawsuit in the country but hasn't been able to collect.
Trump tweeted, "When will the US stop sending $'s to our enemies, i.e. Mexico and others."
From the US standpoint, though, the bigger question is how this benefits Trump.
In the past, Pena Nieto has criticised Trump and his proposals. In March, Pena Nieto compared Trump to Adolf Hitler and Benito Mussolini, saying that Trump's "strident" rhetoric was of a piece with the arguments those leaders used to gain power. Pena Nieto has also flatly rejected Trump's signature policy proposal, to build a wall on the border and have Mexico pay for it. Not going to happen, Pena Nieto said to Fareed Zakaria of the Washington Post and CNN: "There is no way that Mexico can pay [for] a wall like that."
After Trump and Pena Nieto meet, that will be the first question that's asked of Trump. Did Pena Nieto agree to pay for the wall? Pena Nieto has a clear political incentive to embarrass Trump on the issue, a sort of I-invited-him-here-to-boss-him-around sort of thing. It's perhaps Pena Nieto's only possible positive political outcome. But even if things progress quietly, it forces the issue: Trump says he'll make Mexico pay, and Mexico says it won't. Now what? Trump has never been able to answer that question.
The move feels a bit like John McCain's decision in September 2008 to suspend his campaign to deal with the economy
One deeply optimistic Trump supporter, former congressman Joe Walsh, figures that Trump obtaining a promise to pay for the wall would be "game, set, match," which is true. It is also true that if Pena Nieto gives Trump proof that Clinton was a space alien intent on destroying the globe that it would benefit Trump. Neither is likely to happen.
What does Trump get out of it? We assume that Pena Nieto will pose for photos with the visiting dignitary (though that seems like a political miscalculation for him). Trump-as-statesman is a new one, and it will be interesting to see how it's handled. That photo itself encapsulates a lot of the risk-reward calculus for Trump: At best he gets a dull picture of himself standing next to a person with whom most Americans aren't familiar; at worst, he gets an awkward picture posing in front of the Mexican flag - something that some part of his base probably won't be thrilled about.
The trip will, at best, show that Trump can go to a foreign country and meet with leaders without incident, a fairly low item on the presidential checklist. (A subject for another time: Do voters actually care about a grip-and-greet?) At worst? Who knows.
The move feels a bit like John McCain's decision in September 2008 to suspend his campaign to deal with the economy.
It felt gimmicky and didn't do much - and reinforced that McCain was in the sort of political position that necessitated gimmicks that might not do much.
Trump's trip to Mexico instills a lot of risk with the potential upside for Trump being that he proved he can do something fairly simple without incident. For Pena Nieto, the potential upside is that he can score points off an unpopular visitor; the downside is that he is seen as embracing someone his constituents vehemently dislike.
Given how low the reward is for Trump and how high the possibility that something might go wrong, there's a decent chance that the politician for whom Trump's trip is beneficial is his opponent.
