WASHINGTON — It can be difficult to notice history as it happens. Pivotal moments are often recognizable only in hindsight; decisions that seem minor at the time turn out to mean everything.

Thursday’s referendum in Britain is different. If Britain chooses to leave the European Union — a “Brexit” (Britain + exit, get it?) — we know with certainty that this decision will change the future of the country and of Europe.

Leaving the union would gouge a hole in Britain’s economy as well as Europe’s. In the longer term, a Brexit could weaken the European Union and give momentum to nationalist politics already simmering across the Continent.

To grasp why Britons might choose the risky path of independence over the safer status quo, you need to understand the deep divisions within the country — and that, for many voters, it is a referendum on the present as much as the future.