Read: The tenacity of Trump

The solution, Trump said, was simple. All it would take was a little common sense, and getting the right person in place. If only there were a president who was willing to be tough on immigration and implement the obvious policies, illegal immigration could be stopped. For more than two years since becoming president, Trump has tried to prove that he was right. He has instituted a long string of policy ideas that he thinks will prevent migrants from entering the United States, and each has—for its own reasons—failed.

The first big idea was one that Trump unveiled in that campaign launch. “I would build a great wall, and nobody builds walls better than me, believe me. And I’ll build them very inexpensively. I will build a great, great wall on our southern border,” he said. “And I will have Mexico pay for that wall.”

Predictably, Mexico did not agree to pay for the wall. Trump then said he’d try to force it to do so through a remittance tax, taking a cut of all money that Mexicans in the United States sent back home, but he eventually stopped talking about that—it was unclear that it would raise enough money, or that it would be workable or legal. Having effectively conceded that Mexico wouldn’t pay for the wall, Trump then tried to get Congress to do so.

Nearly as predictably, Congress did not agree to pay for the wall either. When Republicans controlled both houses of Congress, GOP leaders were reluctant to cough up the cash. Trump, for reasons that remain unclear, decided not to push them on it. Then came the 2018 midterms and the Democratic takeover of the House. He decided to get tough only then, forcing a government shutdown for more than a month, from December 2018 to January 2019.

Read: Trump’s bizarre, rambling announcement of a national emergency

Trump eventually blinked. Then he announced plans to pay for wall construction by reprogramming already allocated funds under a national-emergency declaration. While some of those moves seem straightforward, others are of questionable legality and tied up in litigation. And even if Trump were to succeed in court, these measures would pay for only a small portion of the wall. Ultimately, he will need Congress.

With wall construction on ice, Trump has tried a variety of other measures to stem the flow of migrants across the southern border. He dispatched thousands of troops to the border, but under the U.S. laws the president invoked, the military can’t enforce immigration laws, so instead service members went to the border, assisted with some construction, and then sat around bored and hot for a few weeks before being withdrawn.

The law has, in fact, proved a powerful constraint on Trump’s border plans. The administration infamously decided to separate children from parents arriving at the border, but was pounded by both public opinion and the courts; meanwhile, there’s little evidence that the move had the intended deterrent effect. Trump has attempted to reshape asylum laws in a variety of ways, though those efforts, too, have been caught up in court, since the president can’t unilaterally change asylum laws passed by Congress. Trump reportedly told border agents to block asylum seekers, contrary to the law, and promised that if they were prosecuted, he would pardon them. (No one seems to have followed this advice.) Trump has also threatened to close crossings, but has been talked out of it by aides.