LONDON — If you have over the last 30 years arranged a pro-Palestinian event of any significant size in the United Kingdom, the chances are that Jeremy Corbyn was there, either as a speaker or in the audience. Mr. Corbyn, who is now the leader of the Labour Party, is an avid and unwavering supporter of the Palestinian cause.

While not all British criticism of the Israeli government is anti-Semitic, it has a tendency to blur into it at the fringes. As a backbencher, Mr. Corbyn shared platforms with the likes of Raed Salah, a convicted anti-Semite, and he praised Hamas. Those former associations have been dogging Mr. Corbyn since he took over the Labour Party’s leadership in 2015.

This summer, Labour’s anti-Semitism problem is once again dominating the headlines in the British press and dividing the party. And as much as everyone would like it to, it doesn’t look like it will go away any time soon. To understand why, you have to understand Mr. Corbyn and what he cares about.

For the most part, Labour legislators are far more interested in domestic matters — the National Health Service, tax reform, education, basically anything — than the world outside of Britain. There are, however, a few exceptions. Mr. Corbyn is one. Foreign policy is his passion.