Straddling East and West, Turkey is a critical ally of the U.S.

The sprawling nation of Turkey is one of the United States’ most important and most critically strategic allies, straddling the divide between the Middle East and the West.

As the only majority-Muslim member of NATO, Turkey has lent its soil to U.S. air bases, supported American military operations in key conflicts — such as Syria today and the Balkans in the 1990s — and served, until recent years, as a rare friendly interlocutor between Muslim nations and Israel.

But Turkey has also been a complicated and prickly ally, and more so as President Recep Tayyip Erdogan deepened his autocratic hold on power.

Turkey’s stability and the friendliness of its military toward the West are also of vital importance to the U.S. and for countries throughout Europe.

Turkey has been a NATO ally since 1952, and U.S. warplanes have used the Incirlik Air Base in the south during the invasions of Afghanistan and Iraq.

An estimated 1,800 U.S. military personnel are assigned to the base and the U.S. Embassy in Ankara, the capital.

With the conflict in Syria, Erdogan was hesitant to partake in military action in the early months of the U.S-led effort against Islamic State militants. For Erdogan, the greater goal was ousting Syrian President Bashar Assad, and Erdogan was accused in some U.S. circles of turning a blind eye towards Islamic State.

However after a series of high-profile suicide attacks in Turkish cities, Erdogan agreed a year ago to allow U.S. warplanes fly combat sorties from Incirlik.

Since then, the intensity of the U.S.-led air war in Syria increased sharply because the flight time into Syria has been drastically reduced compared with using other, more distant U.S. bases. The Pentagon in March ordered military family members to leave Incirlik due to a uptick in terror attacks and the risk that they posed to Americans at the base.

Turkey has also begun to clamp down on smuggling routes along its 500-mile border with Syria that Islamic State militants use to move fighters, money and weapons -- especially along a porous 60-mile stretch known as the Manbij Pocket.

Thousands of foreign fighters have slipped across the border amid the maze of supply lines that go through Turkey to join the various militant factions in the multi-sided Syrian war.

The U.S.-led coalition, with Turkey’s help, is in the midst of a massive, months-long operation to close the Manbij Pocket. Since the operation began, coalition warplanes have launched about 400 airstrikes to support ground forces known as the Syrian Arab Coalition to push the last remaining Islamic State fighters from the area.

The complications for Turkey, however, and Erdogan especially, persist. Erdogan sees Turkey’s greatest enemy as Assad, who has generally managed to hold on to power, and the array of armed Kurdish forces, some of whom have proven to be the United States’ best-trained allies in fighting Islamic State. Erdogan sees most of the armed Kurdish groups as terrorists and chafes at their relationship with the U.S., which only regards one faction -- the Kurdish Workers Party (PKK)-- as a terror group.

Instability in Turkey introduces yet another element of uncertainty into the U.S. fight against Islamic State, Nicholas A. Heras, a Middle East researcher and the Bacevich fellow at Center for a New American Security, a think tank in Washington.

“We don’t know if a coup government would honor the agreement the U.S. worked out with Turkey” which has allowed the U.S. military to work with Kurdish militants in northern Syria to push back Islamic State fighters, Heras said. The Turkish military has fought against a Kurdish independence movement inside the country for years and getting Turkish officials to allow the U.S. to arm and train Kurdish militants in northern Syria was a major diplomatic challenge. A new set of leaders in Turkey could upset that balance, Heras said.

“The ball would be in the court of the coup government, they would have to determine if they would continue,” he said.

One major challenge will be whether the U.S. could continue to provide billions in military and counterterrorism funding to Turkey under a new government. Under U.S. law, if a coup government took power, the U.S. wouldn’t be able to provide military aide, Heras said.

“Turkey is the essential state in the counter ISIS fight on the ground,” Heras said, using an alternative acronym for Islamic State. Getting locked out of the Incirlik base would be a major setback in the U.S. effort against Islamic State.

Turkey has also been instrumental in absorbing the brunt refugees from Syria and Iraq. More than 3 million are believed to be living in Turkey, and many more cross its borders.

As such, Turkey’s geographic position and its flexible government have served as something of an escape valve for besieged nations on both sides of the exodus.

Geography both blessed and cursed Turkey. As the vast Ottoman Empire, what would become Turkey was for centuries a prosperous crossroads for merchants bearing silk and spices and other riches, flourishing under Suleiman the Magnificent in the 16^th century.

And it was the site of brutal battles of conquest and territorial supremacy waged with Persia and the Christian armies of Spain and Venice.

Modern Turkey emerged from the ruins of the Ottoman Empire and the ashes of World War I. A nationalist leader named Mustafa Kemal, known as Ataturk, or “Father of the Turks,” molded the nation, and its military, based on a staunchly secular Islamic faith and a deeply held Turkish national identity.

Erdogan’s recent move toward a more prominent presence of Islam in Turkish life and in the government may have made the army increasingly nervous. The army has also been empowered by Erdogan to fight, largely carte blanche, a large and restive Kurdish minority, some of who have taken up weapons against Ankara.

Staff writer Brian Bennett in Washington contributed to this report.