Should the US pull its nuclear weapons out of Turkey?

"Welcome to Incirlik AB, Turkey," reads the big gold letters over one of the entrances to a key staging base for the fight against the Islamic State, strategically located in southern Turkey, just 70 miles from the Syrian border.

But this month, Germany learned that the friendly greeting is not written in stone, and that the NATO ally can pull in its welcome mat with little notice or explanation.

After the government of Turkish President Recep Tayyip Erdogan refused to guarantee that members of the German parliament could visit German troops at Incirlik, Berlin announced it was moving its forces and planes to Jordan.

Germany's decision to decamp to more friendly territory starkly illustrates the growing tensions between Turkey and some of its NATO allies, including the United States.

"When our major European ally pulls its forces out of Incirlik because it couldn't be guaranteed access, it should warn you what happened to Germany could happen to us," said Joe Cirincione, president of the Ploughshares Fund, a Washington think tank focused on nuclear weapons policy.