Human memory tends to perceive minor events as brief and major events as lasting, which warps our perception of time’s passage. Perhaps that’s why it feels like it’s been an eternity since President Donald Trump fired FBI Director James Comey, when in fact it happened just a year ago today.

The significance of Comey’s ouster was clear when it happened. What the public learned in the days and weeks that followed only confirmed its importance. Though Trump is through only a third of his first term, Comey’s firing looks to be the signature mistake of his presidency, akin to Bill Clinton declaring he “did not have sexual relations” with Monica Lewinsky or George W. Bush prematurely declaring “mission accomplished” in Iraq.

Many observers, including me, have written about what Trump’s decision and its aftermath meant for American democracy and the rule of law. Not since Richard Nixon during the Watergate crisis had a president so brazenly interfered in a federal investigation into himself and his associates. But Comey’s firing also came with tremendous costs to Trump—to his family, his allies, his presidency, and ultimately himself. Those costs may be far more important in the long run.

Start with the obvious legal consequences. Trump spent his first months in office pressuring Comey to do two things: drop an investigation into one of Trump’s top political allies, and prematurely declare the president himself wasn’t under investigation for collusion with Russia. Comey rebuffed him. After Trump fired Comey and then publicly threatened to release tapes of their conversations, Comey responded by leaking memos that detailed Trump’s efforts. Those revelations effectively forced Deputy Attorney General Rod Rosenstein to appoint Robert Mueller as special counsel to lead the Russia investigation.

Trump’s surreptitious efforts to exonerate himself and spare a political ally from prosecution failed spectacularly. Flynn, as part of a plea bargain with Mueller, pleaded guilty to lying to the FBI in December. What’s more, the episode deepened the president’s legal peril. Top Justice Department and FBI officials remain silent about whether they’ve found evidence that the Trump campaign colluded with Russia to undermine Hillary Clinton, but if nothing else, Mueller could build a solid case that Comey’s firing amounted to obstruction of justice—an impeachable offense.