Trump sent second letter to Erdogan threatening sanctions over S-400s
Letter set out US conditions for resolving crisis over Russian-made missiles ahead of White House meeting, but Turkish president was not convinced, sources say
Erdogan’s bodyguards reportedly also attacked US Secret Service members in 2017 (video)
Court documents give new details about controversial May 2017 incident as Trump prepares to meet Erdogan in Washington this week
Turkey may purchase Russian warplanes, says Erdogan
Erdogan: "Turkey did not ask anyone permission when buying the Russian S-400 missile system nor will not do so if it decides to acquire Russian fighter jets"