Until recently, Scotland's independence referendum received little attention in the United States. Americans had company in London, where political elites also paid little heed to the possible break-up of their country. Then, at the end of August, just weeks before the September 18 vote, polls detected a sudden rise in pro-independence ‘Yes’ sentiment. ‘No’ is still favored to win. But the result will be uncomfortably close.

The ‘Yes’ side has received a boost from a manipulative campaign by Scotland’s independence-minded local authorities. Back in 1998, Tony Blair’s government devolved considerable power to a newly created Scottish parliament. The separatist Scottish National Party (SNP) gained the first ministership in 2007 and won an outright majority in 2011. Tapping into this legislative power, the separatists have rigged the referendum rules in their favor.

Scots are a famously mobile people. “The noblest prospect which a Scotsman ever sees is the high road that leads him to England,” Samuel Johnson once gibed. Those Scots who have taken the road, however, forfeit their vote on whether that road shall remain open or be closed: Anyone who has resided outside of Scotland for more than a year is disqualified from the referendum. The franchise was extended downward to 16- and 17-year-olds: Teenagers, unsurprisingly, are more attracted to the high-risk, low-reward independence proposition than are grownups. The Scottish separatists deployed the resources of government to produce a hefty white paper on independence with blithe assurances that an independent Scotland will enjoy lower taxes, higher social benefits, and less public debt—and that all difficult problems, including currency questions, will be amicably negotiated to Scotland’s advantage after a ‘Yes’ vote.

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.

Fifth, a ‘Yes’ vote would only further encourage German domination of the European Union. The EU originated as a bloc of three large countries (France, Italy, and West Germany) and three smaller ones (Belgium, Luxembourg, and the Netherlands). West Germany ranked as first among equals, but France and Italy could make their voices heard too. Today, an expanded EU contains 19 members that are less populous than Belgium and one—Malta—that is even smaller than Luxembourg. Meanwhile, united Germany looms bigger and richer than ever, accounting for more than 20 percent of the EU’s gross domestic product. Scotland as a 29th member nation (and, who knows, perhaps Catalonia as a 30th), along with the emergence of a weakened United Kingdom, would push the EU in an even more lopsided direction: The median EU member by population would be Austria—a country of 8.5 million that sells 30 percent of its exports to Germany.

Germany is an important American ally, and there is nothing sinister about the role it’s currently playing in Europe. But Germany’s interests do not align precisely with those of the United States or other EU member states. Germany usually favors a more deflationary monetary policy and a more accommodating policy toward Russia than most U.S. administrations and many EU members would prefer. As the ranks of the small, Germany-beholden states of Europe proliferate, the EU is evolving away from its historic role as a restraint on German dominance within Europe—and toward a disturbing new role as a multiplier of that dominance.

In February 1995, Bill Clinton traveled to Ottawa to speak in favor of Canadian unity. “In a world darkened by ethnic conflicts that tear nations apart, Canada stands as a model of how people of different cultures can live and work together in peace, prosperity, and mutual respect,” Clinton told the Canadian Parliament. The U.S. president was a more popular figure in Quebec than that province’s own politicians, and his words likely contributed to the narrow margin of victory of the ‘No’ side in Quebec’s second and final secession referendum later that year. President Obama has played no equivalent role in the debate over the survival of America’s close ally, the United Kingdom. If the ‘Yes’ vote prevails on September 18, Obama’s omission should be remembered in the postmortem assignment of blame for a potential disaster for the peoples of Britain, Europe, and the Western alliance.