British Labour Party leader Jeremy Corbyn, center, launches a new poster for the Labour 'In for Britain' campaign to remain in the European Union, in London on June 7. (Facundo Arrizabalaga/European Pressphoto Agency)

Jeanne Shaheen, a Democrat, represents New Hampshire in the U.S. Senate, where Lindsey Graham, a Republican, represents South Carolina.

As members of the Senate Armed Services Committee, we value Britain as the United States’ strategic bridge to Europe, and we’ve been outspoken in urging the British people to vote to stay in the European Union in this month’s referendum.

The United States has a profound national interest in Britain remaining a member of the E.U., and in NATO remaining the cornerstone of our global security architecture, especially in the face of a newly aggressive Russia.

To say that Britain and the United States have a “special relationship” is an understatement. For more than a century, American and British soldiers have fought and bled together in battle. Together, we have forged the world’s strongest democracies, defeated the mightiest totalitarian empires and anchored NATO, the most successful defensive alliance in history. Our partnership is critical to U.S. national security and to the global order in which the United States and Europe are preeminent.

We cannot think of a group or nation that presents a threat to France, Germany and the United States that doesn’t also present a threat to Britain. When we closely examine the United States’ friends and foes, they almost perfectly overlap with those of our major European allies. The commonality of interests regarding national security threats is overwhelming, and it is our strong belief that there is security and safety in numbers. Going it alone would, we believe, present serious national security challenges for Britain, and we would lose a valuable partner and voice that has been integrated in our common European defense strategy.

It is in our mutual economic interest for Britain to continue to lead from within the E.U. With nearly half of its exports going to Europe, Britain needs a strong say in setting the rules for trade with the nearly 450 million consumers across the English Channel — and this includes being party to the Transatlantic Trade and Investment Partnership being finalized between the United States and the European Union. Likewise, the United States benefits from trade with the E.U., the world’s largest economic union. The E.U. generates trade flows with the United States of nearly $2.7 billion a day and accounts for nearly 7 million U.S. jobs. No one should discount the E.U.’s role in creating U.S. jobs and providing a rich market for our goods and services.

The national security reasons for Britain to stay anchored in the E.U. and NATO are equally compelling. The United States agrees strongly with Britain’s views on combating terrorism, fighting nuclear proliferation and sanctioning Russia for its aggression in Ukraine. We take heart knowing that our friend is advocating for these positions at E.U. councils and summits, where the United States is not at the table.

A British exit from the E.U. would open a Pandora’s box of new problems for Europe. Already, European cohesion is being undermined by terrorism, the migration crisis and Russia’s aggression and political meddling. A British vote to leave the E.U. would likely trigger a second referendum on Scottish independence, which, if successful, would weaken Britain and strand its Scotland-based submarine nuclear deterrent. Moreover, secession could be contagious, with other countries following Britain out of the E.U. and separatist movements in Spain and elsewhere gaining new momentum.

We understand the temptation for Britain to pull up the drawbridge and try to separate itself from Europe’s problems. On this side of the Atlantic, we have our own isolationist, “America first” voices. But history tells us that isolationism is a dead end. For centuries, Britain ensured its independence by engaging with Europe, including intervening against Napoleon and Kaiser Wilhelm II. Then, in the 1930s, as the wolves devoured Europe, both Britain and the United States initially opted for isolationism. We chose a separate peace and got a cataclysmic war. We are indebted to the generation of American and British leaders who, as close allies, led our countries through that conflict and then helped build the multinational institutions, notably the E.U.’s precursors, to prevent its recurrence.

Today, once again, Europe is threatened. Terrorists have launched barbaric attacks in Europe’s great capitals. Russia is attempting to dismember sovereign European countries and sow division across the continent. All of which makes this the worst possible time for Britain to exit the European Union.

Britain is an island geographically but not geopolitically. Today more than ever, its own prosperity requires a successful and secure Europe. “Europe today is facing a series of grave security challenges,” said 13 of Britain’s most senior former military commanders in a recent statement. “Britain will have to confront these challenges whether it is inside or outside the EU. But within the EU, we are stronger.”

We agree. The United States is — and must be — committed to a Europe that is whole, free and at peace. We rely on the E.U. and NATO as the bedrock undergirding Europe’s economic success and national security. This bedrock is far stronger with Britain as a member. Later this month, people in Britain will decide for themselves whether to stay in or leave the E.U. Respectfully, we urge our friends to stay.