As President Obama approaches the tail end of his presidency, his legacy will be marked by many successes and uphill battles, from his plans to reform U.S. immigration legislation that has essentially gone nowhere to a Trans-Pacific trade deal that has stalled in getting congressional approval. With time, a number of his initiatives will probably get higher grades. But it is what he does now to deal with the potential disintegration of America’s most important ally, Europe, that will likely count more.

Historians rank Franklin Roosevelt and Harry Truman among the top 10 American presidents. There are a number of reasons for that, but several that stand out are particularly relevant today and center on what they did for and with Europe. The Lend-Lease Act helped save Great Britain from defeat. The Marshall Plan restored Europe’s recovery from complete devastation from World War II. And Truman’s aid to Greece and Turkey met the threat of Russian expansion.

There is a big difference today. Britain’s exit from the European Union was unexpected. The follow-on problems―potential votes in Scotland and Northern Ireland to dismantle the United Kingdom—are due to unhappiness with the vote in England to leave the EU, and pressures in the Netherlands and France to leave the EU are due to internal dissatisfaction with Brussels. External threats to Europe are far clearer calls to action than trying to address others’ domestic political issues. One could argue that how the EU of 27 deals with the U.K. as a result of the breakup (France for harsh treatment, Germany seeming more understanding) is its business―not the U.S.’s. But what happens next is central to America’s national security.

Europe does not need a new Marshall Plan and Britain does not need a new “lend-lease” program. And Eastern Europe doesn’t need an emergency economic aid package to meet further Russian expansion. But the U.S. needs a strong Europe, and in making this outcome more likely, the U.S. can―and should―play a substantial role.

The prompt offer of a reciprocal tariff-free trade agreement to the U.K. and to the EU of 27, together as one—done quietly so as not to provoke a backlash of foreign interference—could in effect buy the United States a seat at the table, making America relevant to their intra-European discussions. More important, it has a chance to restore stability to Europe.

President Obama and congressional leaders have the opportunity to join together as they never have before―this time to meet the challenge of the potential loss of America’s primary partner in world affairs. Any U.S. presidential frontrunner should be thinking about how the United States can keep Europe together, not applauding its disintegration. It is more than a geopolitical issue. If the threat remains that trade barriers will rise between the U.K. and continental Europe, there is a strong likelihood that uncertainty for business will cause world growth to slow. That is not good news for any country.

Removing tariffs with Europe will not be as politically controversial on either side of the Atlantic as a more in-depth trade and investment agreement, and is not as complicated. It is a step that can be taken quickly. Wage rates in Europe and the United States are not dissimilar. The gains will be mutual.

An agreement of this sort will not be embraced overnight, and probably would not be concluded in the next six months. But negotiations to achieve it could be launched. Doing so just might prove to be the most important initiative that future historians will point to in ranking Obama and this Congress.

Theodore Roosevelt―another memorable president―received a Nobel Peace Prize in 1906 for mediating the end of the Russo-Japanese War. President Obama got his Nobel Peace Prize at the beginning of his first term.

Mediating a good outcome to the Brexit-induced crisis, and ultimately maintaining stability in Europe, would be much more important to the United States than what Theodore Roosevelt did to earn his award.

Maybe the U.K. and the EU of 27 will solve this on their own, but America has too much at stake to rely solely on that happening. A first step would be to make an offer that Europe could consider. Success in helping to keep Europe together would make the history books.