Sen. Mike Lee (R., Utah) threatened to sabotage Japan’s security agreement with the United States unless the prime minister frees an imprisoned Navy officer by the end of February.

Lee took to Twitter to bash Japanese prime minister Fumio Kishida for holding Lt. Ridge Alkonis, who was sentenced to three years in prison for a fatal car crash in Japan in 2021, which Alkonis says occurred after he lost consciousness due to altitude sickness.

“Let’s face it, @kishida230, you’ve got a really good security arrangement with the United States, and you’ve had the luxury of not having that arrangement discussed or seriously questioned in Congress for a long time. That’s about to change,” Lee wrote.

The comments—which Lee made on Thursday from his personal Twitter account @BasedMikeLee—are a rare show of force for the senator, an isolationist-leaning Republican who earlier this week objected to the Pentagon’s defense spending and has opposed sanctions on Russia.

“If you transfer Lt. Alkonis back to the U.S. before midnight on February 28, 2023, we will do our best to forget that this whole thing never happened. It will be hard, but we will try.”