Brexit tore the United Kingdom and the European Union apart. Now, President Trump’s America is prodding them back together.

Five years after Britain formally exited the European Union, their leaders are moving cautiously toward a new era of cooperation. In an event that has been billed as a major reset of relations, the two sides will meet for a summit on Monday in London.

They are expected to announce a landmark defense and security agreement intended to bolster military security in Europe as the United States downgrades its commitment to the continent. Negotiators could also announce plans to relax some rules on food trade and to deepen energy cooperation.

Admittedly, reaching agreement on the shape of the new relationship has not been easy. Despite the high stakes on defense, more prosaic concerns, including fishing rights and a plan for youth travel between countries, have remained painful sticking points for negotiators.

But Monday’s event aims to underscore the commitment of the two partners to working together during an increasingly fraught geopolitical era. Hanging over the broad agenda will be one overarching theme: how to push ahead in a world where the United States is pulling back.

After decades of global leadership on defense and an embrace of free trade, the United States’ role in the world is being fundamentally reworked by the Trump administration. Mr. Trump has been pushing for Europe to shoulder more responsibility for defense. He has moved away from full-throated support for Ukraine, and has drawn rhetorically closer to Russia. And he has waged a trade war on the world, slapping diplomatic friends and foes alike with much higher tariffs.