Donald Trump is closing in on the American presidency.

The man who has promised to build a wall on the Mexican border (and force the Mexican government to pay for it), to somehow or another destroy the Islamic State with one fell swoop, to push Vladimir Putin back where he belongs and to rectify the trade imbalance with China is on a trajectory to win the nomination for the Republican Party of the United States.

And that’s worth considering.

What would a Trump presidency be like? For all his bluster, he’s remarkably short on details: even Barack Obama as early as 2007 indicated pretty clearly he’d begin a drone war in Pakistan. Beyond Trump’s obviously offensive platitudes and incoherent rambling about both loving and hating large swathes of people (sometimes the same people), Trump has kept actual policy close to his chest, if he has any at all.

But that doesn’t actually matter so much when it comes to speculating a Trump presidency, because Trump, like all presidents, must play by the rules.

Chief among them: No American president defines America and no president ever will

American presidents have powerful constraints that keep megalomania in check. They are doomed to eight-year stints, bound by a strong, time-honored that is remarkably difficult to change. Bad presidents need not be overthrown, simply outlasted. Presidential enemies tick down the days and throw their energy into the next election rather than sedition and civil war.

This prevents any one president from ever lasting long enough to establish the kinds of cults of personalities necessary to truly alter the US. Even the Bush family, a political dynasty if ever there was one, had little continuation between father and son, with the two presidencies remembered as vastly different affairs.

This also means presidents have only so much time to use so much power. In political science terms, it’s called “political capital” — the mushy social mass of influence, bribes, promises, lies and goodwill that propel people to obey the president. Eventually, all presidents run out of capital.

President Obama, for instance, came in with a great deal of political capital in 2009: He was elected by a wide margin, controlled both houses of Congress and had little baggage he could be attacked on. But despite these advantages, Obama used up most of his capital within a year enacting his health-care reform, known colloquially as Obamacare.

That one achievement was huge: to restructure America’s health care insurance industry is to change $3 trillion worth of the economy, or about 17 percent of the total US GDP.

From there, Obama’s achievements become more mundane. Having spent virtually all his political capital on Obamacare, he has had to content himself with small tactical victories that, while they certainly will define his presidency, have not permanently changed the US.

Take gay marriage; that was the action of the American Supreme Court. Gun law reform? A handful of executive actions that may well expire with his presidency. Climate change? Largely on hold.

Compare this to his predecessor, George W. Bush, whose cornerstone achievements were tax cuts and the Iraq War. Bush tried to reform immigration and Social Security, but got nowhere because his capital had already been spent on his tax cuts and on fighting the Iraq War.

In other words, most presidents get one, two big things they change, a handful of small things they alter and a bunch of stuff they never get anywhere close to touching.

But why?

It isn’t merely that America is cleverly divided against itself: the legislature and the courts balance the president. It’s also because of the vast size of the United States: to enact sweeping change, the president must first communicate said plans to the nation, build political momentum and shepherd policy into being. All of that is wholly consuming: media outlets demand interviews, citizens demand assurances, politicians demand favors. The more controversial the topic, the more energy it takes.

To communicate a message to tens of millions of voters is a massive task. To get them to agree, let alone organize to push their politicians to do so as well, is even more difficult. This is a key reason why smaller democracies can seemingly switch lanes so quickly: nationalizing health care in tiny Belgium was never as complicated as Obamacare.

So knowing that Trump could accomplish only one, two big things, and then a few more small things, what would he likely go for?

Trump’s single concrete policy proposal is his Mexican wall — a policy that is half-doomed to fail. Let’s assume that a Trump presidency accompanies a friendly Republican Congress as well; getting funds for a wall would be relatively easy. But getting Mexico to pay for it? This is where Trump would fail first.

Trump is assuming nation states are like businesses in which big, rich, important businesses get to set the terms of relationships. This can be true, but nation states are not profit-based: elites are motivated by power, which is given by popular consensus. And it goes without saying that Mexicans have a very low opinion of said wall.

Any Mexican politician who stands up to Trump will earn a windfall of easy popularity. Far from cowing Mexico with tough talk, Trump will actually empower his enemies within Mexico. Trump could not threaten Mexico with war: the American public would never countenance that. Nor could he enact sanctions or cut ties to America’s southern neighbor: the economic impact would be so painful Congress would never consider it. So he’d be stuck moaning about how useless the Mexican government is while watching anti-Trump Mexican politicians take power.

After that, Trump would likely try to begin mass deportations: this would doubtless be enacted with gusto by a friendly Republican Congress. But the sudden removal of illegal immigrants would also destroy a cheap labor pool. Prices would rise in sectors dependent on illegal immigrants; a political backlash that would siphon Trump’s capital would follow.

After that, it would be very likely Trump could only accomplish small things

Weakened after the debacle with Mexico, Trump would lurch from crisis to crisis, miscalculating on the global stage. Trump would, like George W. Bush before him, eventually learn that tough talk ends up empowering American enemies rather than cowing them. Disciplined opponents like Russia and China would exploit missteps.

Russia, for example, would use a Trump presidency as a chance to weaken both NATO and the EU. Europeans are rightly horrified of a Trump presidency: Trump would be likely to drag the conflict with Russia to a height few Europeans want. This deep unpopularity would be a chance by Russia to weaken the NATO alliance: key targets would be Germany and France, though Trump’s anti-Muslim rhetoric might win him allies in post-Paris attack France.

That being said, if Trump deployed American power to Europe in vast quantities, he could well put a stop to Putin’s expansionism. Putin might see Trump as so unstable that he’d no longer be willing to take risks that might escalate to nuclear war. Obama is more reliably sane: Donald Trump assures few on that front.

Meanwhile, Trump would need to make a show of war against the Islamic State but would find it a tough proposition to fight a big war against it. Having already used up most of his capital cracking down on immigrants and building his wall, Trump would focus on propaganda victories rather than actual ones: blustering about airstrikes would probably be his bit. Despite the growing Islamaphobia in America, few Americans want to return tens of thousands of troops to the battlefields of Iraq.

It would be with China that Trump would accomplish the least. While China is certain to have a troubled decade ahead of it, Beijing’s foreign-policy discipline would thrive against Trump’s erratic behavior. As Trump might seek to win the news cycle and dominate headlines, he’d seek tactical, short-time wins at the expense of long-term solutions. Rather than appear the reasonable power in the Pacific, he might well drive Pacific states to see China as the more responsible of the two countries. This could undo much of the Pacific’s security and economic architecture as states scramble to either accommodate China or chart a path independent of both America and China.

Japan, for example, might well take its rearmament a step further: seeing Trump’s America as more a liability than an asset, Japan might decide to emerge as a great power once again to challenge both the United States and China.

Meanwhile, it would be highly likely that Trump’s populism would wear thin and that his presidency would suffer losses and eventually a defeat

Trump lacks the skills of a successful president and we’ve yet to see any advisors who will save him from himself. It is reasonable to assume he’d quickly lose Congress to the Democrats and that a second term would be seriously in doubt. As Americans see up close what a Trump presidency actually means, they would be organized and energized to halt his policies. Some of that would come from Republicans themselves: Trump is not a committed conservative and would doubtless veer from issue to issue as the popular winds blew. This would endear him to few.

Eventually, such forces would catch up with him: in the hyperpolarized United States, impeachment would always be just a few steps away. If he made it to a second term, it would be as a wounded animal, unable to do much beyond maintain his Mexican wall and provide horrifying soundbites. There would be some solace in that.

This story first appeared at Geopolitics Made Super, February 24, 2016.