The standard political expectation is that the ‘No’ side of any referendum will outperform its poll-measured strength, as natural voter caution asserts itself at the last minute. But Americans need to understand: Their national interests are also at risk on September 18.

A vote in favor of Scottish independence would hurt Americans in five important ways.

First, a ‘Yes’ vote would immediately deliver a shattering blow to the political and economic stability of a crucial American ally and global financial power. The day after a ‘Yes’ vote, the British political system would be plunged into a protracted, self-involved constitutional crisis. Britain’s ability to act effectively would be gravely impaired on every issue: ISIS, Ukraine, the weak economic recovery in the European Union.

Second, a ‘Yes’ vote would lead to a longer-term decline in Britain’s contribution to global security. The Scottish separatists have a 30-year history of hostility toward NATO. They abruptly reversed their position on the military alliance in 2012 to reassure wavering middle-of-the-road voters. But the sincerity of this referendum-eve conversion is doubtful. Even if it was authentic, the SNP’s continuing insistence on a nuclear weapons-free policy would lock U.S. and U.K. forces out of Scotland’s naval bases. The SNP’s instincts are often anti-American and pro-anybody-on-the-other-side of any quarrel with the United States, from Vladimir Putin to Hamas.

Third, a ‘Yes’ vote would embitter English politics and empower those who wish to quit the European Union. Since the 1990s, the central British government has attempted to appease Scottish separatism. Tony Blair devolved powers; David Cameron agreed that the U.K. would recognize a Scottish independence vote as binding. In the wake of a ‘Yes’ vote, however, English public opinion would harden. The bargaining over public debt, ownership of North Sea oil, and other contentious issues would be ferocious—and those English politicians who urge a tougher line on these matters would likely dominate the debate. Such politicians also tend to be Euro-skeptics. The United States has traditionally preferred an EU that includes the U.K., both because a cross-Channel common market makes it easier for U.S. businesses to conduct commerce and because U.K. leaders—from the Conservative and Labour parties alike—have historically pushed the EU in a more free-market direction.

Fourth, a ‘Yes’ vote would aggravate the paralysis afflicting the European Union. An independent Scotland would seek admission to the EU as a 29th member state. A club of so many member states cannot function by consensus, and the EU has yet to develop more effective decision-making methods. The result, much of the time, is that no decision is made at all—a dynamic that Vladimir Putin depended on when he picked a fight with a multinational entity that is notionally much richer and stronger than Russia is.