HAMBURG, Germany — Angela Merkel is the chancellor of Germany, not the prime minister of Britain, but it’s becoming increasingly clear that she played a critical, if indirect, role in Britain’s 2016 vote to leave the European Union. If not for her decision not long before the vote to allow for uncontrolled mass immigration into the heart of Europe, the pro-Brexit forces might have lost.

Remember that, like President Trump’s election that same year, Brexit was dismissed as an impossibility, until it happened. But unlike in the United States, Europe has not done much in the way of asking what happened, and why.

In late 2015, the Leave campaign started putting up placards which showed the exodus of refugees from Syria and other countries through the Balkans, and adorned them with slogans like “Breaking Point” and “Take Back Control.” With Ms. Merkel declaring an open-door policy, the message hit home for millions of worried Britons and Europeans. Not coincidentally, it was around this time that support for Brexit began to tick up.

If members of the chancellor’s government recognize her complicity, they don’t show it. Ms. Merkel’s diplomats in Berlin treat the looming farewell of the second-biggest economy in the European Union as if it were merely another tiresome bureaucratic process that needs to be handled according to the treaty rules. If they consider the bigger picture at all, it is to think of ways to scare off other union members from following Britain’s lead.