President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE has presented Americans with a clarifying moment. Should the United States retreat into an “America First” isolationist shell, or should it remain engaged with the world? Trump has set the retreat in motion with his decision to declare victory in Syria and withdraw, as well as to withdraw from Afghanistan for no clear reason whatsoever. The isolationists who have condemned the United States involvement in the Middle East and rest of the world for decades are about to get their wish. We will witness what the world looks like when left to its own devices.

But liberal and conservative internationalists must now make up their minds. Shall we continue with the mutual accusations, partisan bickering, and self canceling rhetoric? Or shall we put aside our policy differences in defense of the principle that the United States must remain actively engaged in the world, for our own interests and those of other countries? The choice is obvious if our nation has the collective will to make it.

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Step one in forming a nonpartisan internationalist coalition is setting aside all “original sin” arguments. One can believe that the invasion of Iraq in 2003 set in motion all the badness that has followed but still recognize the need to work in defense of internationalism with those who advocated it. One can similarly believe that the withdrawal from Iraq in 2011 and the Iran nuclear deal did incalculable harm to our interests, yet commit to joining with the architects of those decisions in the fight to keep the United States engaged at all. The past is past. The mutual recrimination society into which American politics has descended only abandons the field to isolationism. The question is not who is to blame for where we are now but rather what should we do in the future?

Step two is recognizing that engagement in the world really does require all instruments of national power. We need a strong State Department, international aid program, and all the tools of soft power to combat the Salafi jihadi movement, Russia, China, Iran, and the chaos that threatens to engulf the world. Republicans should support the budgets for those programs and demand that they be integrated into all security strategies.

We also need a strong military that is able to face the wars we are still in and preparing for the wars of the future. For, to paraphrase Leon Trotsky, you may not be interested in war, but war is interested in you. Democrats should recognize that dissatisfaction with the way in which past and current presidents have wielded the military instrument does not mean that the military is unnecessary against opponents who use it freely.

Step three is seeing that vigorous and even ferocious debates about exactly how the United States should engage in any particular area are healthy, as long as they do not descend into political assaults on the motivations of the disputants or undermine the commitment to some kind of engagement. We will not agree on precisely what to do most of the time. We must accept that liberal internationalists will win some debates, conservative internationalists will win some debates, and that is fine, as long as isolationists do not remove America from the world stage entirely.

Step four is accepting how bad the world situation is now, how much worse it will become, and how high the price of righting it will be. The withdrawal of the United States from Syria opens the door to full scale competition and war between Russia, Iran, Turkey, Israel, Kurds, Arab states, and the Salafi jihadi movement. That war will blow hot and cold. It will then expand to engulf the region. It will fuel the global Salafi jihadi movement, funnel resources to Iran that undermine efforts to pressure Tehran, threaten Israel possibly to the point of triggering a major conflict with Iran, and fuel Arab Iranian tensions possibly to the point of open war.

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The withdrawal from Afghanistan will likely lead to the collapse of the Afghan government and the return not merely of the Taliban but also of Al Qaeda as well, and it will certainly lead to the growth and expansion of the Islamic State franchise there. The implications of American global retreat will encourage Vladimir Putin, already massing forces to invade Ukraine again if he so chooses, to expand his efforts to destroy the North Atlantic Treaty Organization and the European Union, regain control of the former Soviet states, and marginalize the United States around the world. They also will encourage China to press forward in its drive to break up the American alliance system across Asia and establish a hegemony centered on Beijing. Since Japan and South Korea are unlikely to accept such a hegemony tamely, this will likely to lead to major war in Asia.

Stopping and reversing these trends, or even some of them, will require a massive American commitment of both military and nonmilitary power sustained over decades. We will have to spend lives and treasures to recreate a world environment in which the United States and our ideals can once again be safe and ultimately thrive. We will not be talking about 2,000 troops here or 10,000 troops there, nor about a few billion dollars in aid, weapons sales and other forms of assistance to partners on the brink of destruction. The cost will be much higher than anything we have seen over the past two decades, which is one of the reasons, of course, why the decision by the president to retreat is so foolish and short sighted.

America will ultimately be able to prevail in this effort. Our people will continue to draw strength from their freedom and ingenuity, a strength of which our autocratic adversaries deprive themselves. Our size and geographic advantages make our defeat improbable, yet those on both sides of the political aisle who see these dangers, as well as the need for America to rebuild a world in which we wish to live, must unite. If we do, we can have every hope of succeeding. If we do not, we are doomed.

Frederick W. Kagan is the director of the Critical Threats Project at the American Enterprise Institute. The author of “Choosing Victory” and one of the intellectual architects of the successful surge strategy in Iraq, he is a former professor at the United States Military Academy at West Point.