Greek PM on White House trip amid Turkey tension
Greek relations with Turkey are at their lowest point in decades
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
U.S says it will withdraw troops from Northern Syria, as Turkey prepares to invade
U.S. troops “will not support or be involved in the operation", the White said
