WASHINGTON – In its battle against Islamic State, the United States relies on a host of players to follow through on essential deliverables.Saudi Arabia hosts and funds camps to train Syrian fighters vetted by Washington. Jordan carries the weight of hundreds of thousands of refugees.European nations, many out of Islamic State’s line of sight, provide funding and military support for the air strikes conducted by the US and five Arab states over Syria.And over Iraq, jets and drones from the United Kingdom, France and Australia have joined the US Air Force.Generally, the 40 countries involved in the coalition have acquiesced to the requests of the Obama administration. They are otherwise replaceable. Budget displacement and sources of funding make some partners expendable or, cast alternatively, pure value added as they enter the long, arduous fight against an organization viewed universally as a terrorist organization.Turkey is the outlier by all these measurements: its government has not followed through on what the US, or any of its other treaty-bound allies in NATO, have come to expect of them. And it is not expendable: Turkey controls the longest border with Syria, making it in part responsible for the flow of foreign fighters into the ranks of Islamic State, and for its direct military response in stopping the group.Relations between US President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan have long been tepid, after a 2010 leak of diplomatic cables revealed US disregard for his understanding of the region and contempt for his “sycophantic,” “isolated” and “fearful” team of advisers.But the US now needs Turkey, and Turkey has demands that would fundamentally change the course of the war.Ankara wants safe havens along its border to put space between itself and Islamic State territory, and a no-fly zone to protect its assets from air forces loyal to President Bashar Assad. It wants humanitarian enclaves set up in newly established safe havens in Syria to alleviate the refugee crisis burdening its south. And it wants the US-led coalition to battle Assad and Islamic State at the same time.Obama rejects all of these requests: Islamic State is the universal threat, to Turkey and the US, he says. Turkey’s military is NATO -trained, well prepared for the fight; and international funding has made the refugee crisis a manageable problem.But Obama’s top advisers, from the Pentagon to the State Department, have suggested an openness to Erdogan’s suggestions.Should Turkey get its way, mission creep will have set in full force: boots on the ground would be required for the establishment of safe havens.