Washington Post:

Understanding Turkey’s rollercoaster relations with the U.S. requires a grasp of advanced military hardware. Turkey says its army, NATO’s second-largest, will take delivery of Russia’s S-400 missile-defense system in early July. President Donald Trump’s administration contends that could help Moscow gather critical intelligence and it’s threatened sanctions that could cripple Turkey’s economy. Turkish leader Recep Tayyip Erdogan is betting his personal rapport with Trump will fend off harsh penalties. But with Congress vigorously against the missile deal, there’s a high risk of punishments that could plunge Turkey into renewed economic turmoil.

1. Why is the U.S. so opposed to the deal?

The S-400, also known within NATO as the SA-21 Growler, has advanced radars and isn’t compatible with alliance technology. Its deployment in Turkey would mark a further advance in Russian President Vladimir Putin’s efforts to engineer a bigger role in the Middle East. Chief among U.S. concerns is that the Russian system could be used to collect intelligence on the stealth capabilities of the U.S. F-35 fighter jet that Turkey is buying and helping to build. There’s history here. Turkey is home to the Incirlik Air Base, used for U.S. operations against Islamic State and, decades before that, the main operating location for the American U-2 spy plane — until American pilot Francis Gary Powers was famously shot down over the Soviet Union in 1960.