Days before the American-led invasion of Iraq, five protesters broke into a British military base, intent on disabling aircraft that were set to be deployed in bombing missions.

It was March 2003, and the group said it wanted to prevent war crimes and protect civilians. Among those who later defended them in court was a 43-year-old human rights lawyer.

His name was Keir Starmer.

In a strange echo, 22 years later, Mr. Starmer would face a similar case, but now as prime minister of Britain.

In June, activists from a group called Palestine Action broke into a Royal Air Force base, sprayed red paint into aircraft engines and damaged the planes with crowbars. Like the 2003 group, the protesters argued that their actions were a justified response to mass civilian harm — this time in Gaza.

Both cases raised serious concerns about the security of Britain’s military bases. But a very different result ensued. While the protesters in 2003 were prosecuted under criminal laws against property damage, in June, Mr. Starmer’s government announced that Palestine Action would be added to its list of banned terrorist organizations, alongside groups including Al Qaeda, Hezbollah and Atomwaffen Division, a neo-Nazi group.

It was the first time in modern British history, according to the government’s adviser on counterterrorism laws, that a protest group that does not call for violence against people had been proscribed as a terrorist organization. The decision has fueled an intense debate over the Starmer government’s attitude toward protest and free speech.