A revanchist Russia, terrorism, turmoil in the Mideast, and the Mediterranean migration crisis all point to the need for a vital transatlantic alliance. Unfortunately, it’s never seemed weaker.

There are multiple factors, including eroding European unity highlighted by last summer’s Brexit vote. Other upcoming European plebiscites are also perilous to continental cohesion. Right-wing movements in France, the Netherlands and Germany may amplify the populist, nationalist sentiments that can be antithetical to European, let alone transatlantic, unity.

But the biggest internal challenge to the Western alliance may come from the nation that created and led it: the United States. During his campaign, President-elect Donald Trump openly questioned the efficacy, if not the necessity, of NATO, and called himself “Mr. Brexit” despite key leaders in Great Britain, Germany, France and other E.U. nations favoring a “no” vote. Although Trump has amended some of his ill-considered comments, uncertainty remains. That’s in part why the Eurasia Group, a global political risk research and consulting firm, named an increasingly “independent America” as its top political risk for 2017.

The transatlantic alliance is weaker than it’s been since the end of World War II, Eurasia Group President Ian Bremmer told an editorial writer. Bremmer predicted “a Trump-led United States that will shrug off the burdens and responsibilities of support for multilateral institutions that the U.S. has created in the economic and political space, as well as the fundamental questioning of the utility of long-standing alliances.”

Russia and China will likely look to test the president-elect and further assert their world views, endangering U.S. and allied interests. In response, Congress should consider a more robust role, despite foreign policy being more of a presidential purview. The Senate should especially scrutinize Rex Tillerson, Trump’s nominee for secretary of state, who as CEO of Exxon-Mobil questioned sanctions on Russia for its illegal annexation of Crimea and destabilization of eastern Ukraine. Confirmation deliberations should not be perfunctory for other foreign and defense policy designees, either.

Members of Congress can assert their views directly, too, as Sens. Amy Klobuchar, D-Minn., John McCain, R-Ariz., and Lindsey Graham, R-S.C., did last month during a bipartisan trip to Ukraine, Georgia, Montenegro, Estonia, Lithuania and Latvia. There they hoped to reinforce U.S. support for NATO, discuss cybersecurity issues and address Russian provocations. Klobuchar told an editorial writer that the senators wanted to “make clear to our allies and make clear to NATO that we stand against Russian aggression.”

It’s best to meet turmoil in the world multilaterally, and it would be a mistake for the U.S. to withdraw from beneficial alliances and institutions.