British Prime Minister Boris Johnson has a big problem. Parliament has blocked him from carrying out Brexit on Oct. 31. It also won't vote for an election that might allow Johnson to make Brexit happen.

So what can Johnson do to break out of his political prison? As I see it, the only obvious solution remaining is to ask President Trump to step up in support of America's closest ally. Were Trump to persuade the European Union to give ground on one critical obstacle preventing parliamentary passage of the withdrawal agreement, Johnson might just be able to scrape together enough votes to see Brexit carried out on Oct. 31.

The big point here is the so-called "Irish backstop" issue. A requirement of the EU to protect its trading integrity, the backstop would introduce border checkpoints along the border between Northern Ireland and the EU-governed Republic of Ireland. While the backstop would only come into force if the EU and Britain failed to agree a long-term trade agreement, many members of Parliament are vehemently opposed to the it, believing it risks the fragile Northern Irish peace and would damage Northern Ireland's economy.

A work-around-the-backstop suggestion has been made involving a prospective EU-British deal to treat the Irish island as a common goods area. The problem is that some European leaders, President Macron of France in particular, are opposed to making any new concessions.

Fortunately, the U.S. has ample means to put pressure on a presently weak EU economy. Whether by holding out on a trade deal, or threatening new tariffs, or penalizing European companies for their engagement with China, Russia, and Iran, the U.S. can affect the EU's calculus. Put simply, it can alter the EU's cost-benefit analysis of whether to make new concessions to Britain in regard to the withdrawal agreement.

The basic point is this: the U.S. needs to support its closest ally. Absent that support, Johnson's government risks implosion, and a deeply anti-American government is likely to rise in its place that will probably never honor Britons' choice to exit the European Union.