Turkey’s decision to purchase the Russian S-400 surface-to-air missile system (also known as the SA-21 Growler) has drawn a lot of controversy. So much so that the United States is on the brink of kicking Turkey out of the F-35 program. According to Radio Free Europe, Turkish President Recep Tayyip Erdoğan is scrambling to avoid that outcome.

The purchase of the SA-21 has raised questions, especially given Russia’s actions in the last few years. There are also concerns that the integration of the SA-21 could compromise the F-35. At the same time, there have been other purchases of Russian systems by American allies that weren’t deal breakers. South Korea acquired T-80 main battle tanks and BMP infantry fighting vehicles. Greece bought S-300 surface-to-air missiles (also known as the SA-10 Grumble) in the 1990s. It should also be noted that Greece has allowed Israel to train against the SA-10.