This is historic. Presidents often vow to wipe out big chunks of their predecessor’s legacies. President Eisenhower was going to take on the New Deal. Ronald Reagan targeted the Great Society. Both backed down. Trump, working with congressional Republicans, hasn’t. He’s eager to deflate Obama’s standing and inflate his own.

Obama and Democrats have made Trump’s efforts surprisingly easy. Obama, you’ll recall, succeeded brilliantly in the first two years of his presidency when Democrats controlled Congress. But once Republicans held the House, Senate, or both over the next six years, he ignored Capitol Hill as much as possible. He spared himself the unpleasantness of compromising with Republicans and instead governed by executive orders and regulations.

Decisions taken by the president alone are vulnerable to being erased by subsequent presidents. And that’s what happened to the pact with Iran. It wasn’t a treaty ratified by the Senate. Democrats used the filibuster to block even a nonbinding vote on it. Trump killed the deal with his signature. That was also all it took to quit the Paris accord on global warming.