The first parts of a sophisticated, Russian-supplied surface-to-air missile system, known as the S-400, arrived in Turkey on Friday.

It was a moment American officials had been anticipating for months. They had warned Turkey that purchasing the equipment would jeopardize the country’s expected receipt of American-made F-35 warplanes. And now they must decide how to react.

Here is a look at how the missile system is threatening to drive a wedge into NATO and what it means about Turkey’s relationships with Russia and the West.

What is the S-400?

The S-400 surface-to-air missile system is one of Russia’s most advanced antiaircraft weapons. It is an upgraded version of the S-300, the Soviet Union’s answer to the American Patriot missile batteries introduced during the Cold War.