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Robert Kagan is a senior fellow at the Brookings Institution and a contributing columnist for The Post.

Of the remarkable things we have learned this election year, the most significant is that the current Republican Party is unfit to lead the country. It has failed the greatest test a political leader or party can face, and failed spectacularly. It has abandoned its principles out of a combination of cowardice and opportunism. It has worked to place in the White House the most dangerous threat to U.S. democracy since the Civil War. And perhaps just as revealing, it has in the process engineered its own suicide. Not only has the party refused to save the country, but also it has proved too helpless, too incompetent and too craven even to save itself.

These are the people we’re supposed to put in charge of the House and Senate for another two years? Whom we’re then supposed to rally behind in the battle for the White House in 2020? No. Not this group. We know too much. We know all we need to know.

The coming years are going to require some courage — not tough speeches, at which Republican politicians excel, but tough and politically difficult actions — on entitlements, on immigration and especially on foreign policy and defense. Republicans used to be able to call national-security policy their strong suit. Can they still? All the tough young senators who railed at the Obama administration for its weakness on the world stage — how tough were they when it came to their own political skins? Not tough enough to take on Donald Trump, even though his foreign policy, such as it was, betrayed many core Republican principles and was in most respects far worse than President Obama’s.

[Donald Trump is the GOP’s chemotheraphy]

After years of railing against the Obama administration’s “reset,” the leading Republican spokesmen on this issue said little and did nothing when their own nominee spoke admiringly of Russian President Vladi­mir Putin and when his closest advisers were discovered to be intimately connected to the Kremlin and found to be lobbyists for Putin’s puppets in Ukraine or Gazprom’s pipeline plans. They were silent when Trump went so far as to urge the Russian intelligence services to hack Hillary Clinton’s emails. These are the political leaders who are supposed to stand up to the world’s real strongmen in Moscow and Beijing. Yet they did not stand up to this bullying would-be authoritarian when all he could do was steal away a few of their voters. They would not risk five points in their primary campaigns to stop this man from becoming commander in chief. They were willing to damage U.S. national interests, as they define them, to avoid a close race. These are the men and women to whom we should entrust the nation’s welfare?

Is the other party any better? On national-security issues, probably not, but unlike in the past, one can only say, “probably.” Given the Republican track record, and not only in this election but in recent years — on Syria, for instance, where leading presidential hopefuls opposed the use of force at a critical moment; or on defense spending, where Republican majorities in both houses have allowed the sequester to stand — the contrast is not as clear as it once was. And as it happens, the present Democratic candidate is as solid on matters of national security as almost any Republican, and infinitely more so than the present Republican nominee — which is one reason so many Republican national-security officials have either come out against Trump or have outright endorsed Clinton. Of former top Republican officials, only Dick Cheney and a handful of others have betrayed whatever principles they once claimed to have and destroyed what remained of their reputations for integrity. And whatever one may think of the relative merits of the two parties, at least this much can be said: In this election cycle, it has been the Republicans, not their opponents, who have worked, and are still working, to hand the country over to someone who they know in their hearts would be a disaster for the nation’s security.

[Republicans deserve their sad fate]

Republicans are no doubt hoping that all will be forgiven and forgotten once the election is over. They can start fresh, begin their next round of attacks, rally the faithful, get ready for the next campaign, treat the whole thing as a bad dream. And perhaps they are right to be cynical, to rely on voters’ short memories or to think they can corral those Trump voters. But perhaps, too, there may be some justice in the world. Maybe some voters will remember. Maybe when those who caved to Trump in 2016 begin their campaigns for 2020, some voters will recall that at a moment of national crisis, those politicians promising strong leadership were too weak, too obsessed with winning elections, too afraid of Trump’s angry faithful to have the steady moral compass, the calmness under fire, the vision in the fog of battle that real leadership demands. And maybe voters at that point will look away from those who self-servingly tried to foist Trump on the nation and will turn instead to the handful of Republican officeholders who had the courage of their convictions and tried to stop him from the beginning. Maybe there will be enough voters willing to reward that kind of genuine political courage, enough to make a difference. When that day comes, the party’s reformation and renewal can begin.

Read more on this issue: Paul Waldman: Are Republicans going to start defecting from Trump? Greg Sargent: GOP nominates dangerously insane person to lead America, then panics The Post’s View: It’s beyond debate that Donald Trump is unfit to be president