One way or another, a major chapter in Britain's history is about to be written. President Trump should help to make sure it's a positive one.

I note this in light of British Prime Minister Theresa May's presentation to her Cabinet on Wednesday regarding her newly agreed deal with the European Union. The deal, which requires parliamentary support, will hopefully set the terms of Britain's withdrawal from the EU in March 2019. But fraught with deep political controversy and risk, May's plan will find resistance from a mix of parliamentarians. There will be those who oppose Brexit on any terms, others who oppose Brexit on May's terms, and those who see this as an opportunity to bring down May's government. Regardless, the simple fact is that May will have an almighty struggle getting this deal out of Parliament and into law.

Assuming the deal offers a mix of British engagement with the EU customs union and a protection of British sovereignty (the two keys to balancing economic prosperity and stability with political rights), Trump should endorse May's plan.

First off, the president can continue with the positive trend he set during his visit to Britain this summer and offer May his political support. This might nudge some Conservative Party MPs behind the prime minister in the belief that the U.S. will support British efforts to ensure the EU remains bound by any deal. The key element here in Trump's power is his ability to dangle the prospect of a trade deal between the U.S. and the EU If the Europeans believe Trump might withdraw from that prospective deal on the basis of their treatment of Britain, they will treat Britain better.

Second, and as an extension of the first point, Trump should pledge to work rapidly to conclude a U.S.-U.K. trade deal that would come into effect following Brexit's fulfillment. This will boost the British investor confidence sentiment while also supporting May's argument against the Labour Party that she is the safest pair of hands when it comes to the economy. It cannot be overstated how significantly interwoven Britain's economic position now is with the EU-Brexit negotiations.

Ultimately, Trump has the opportunity here to support America's closest ally at a moment of its greatest need. For reasons of history and alliance, he should do so. The alternative is to sit idle as Britain navigates a harsh economic and political storm. And in that idleness, to allow for the election to power of an ardent anti-American.