Had it not been for Syria's brutal civil war, the tomb of Suleyman Shah would have likely remained a geographical footnote. But this week the 700-year-old tomb became a flashpoint that helped prod Turkey into entering the battle against the Islamic State (IS; also called ISIS or ISIL).

For centuries the tomb has stood beside the banks of the Euphrates, the river where Suleyman is presumed to have drowned in 1236 while on campaign in what is now Syria.

As the grandfather of Osman I, founder of the Ottoman Empire, his tomb and its accompanying shrine are revered historical sites for the Turks.

One of the clauses in the 1921 Treaty of Ankara, which shaped the boundaries of modern Syria, stipulated that the former Ottoman tomb would remain a Turkish exclave within the new Syria, flying the Turkish flag and protected by an honor guard of Turkish soldiers.

And so it has remained. Even when the tomb itself was physically shifted 50 miles (80.5 kilometers) upstream in 1973 to accommodate flooding from the Assad Dam, its essential Turkishness was never in doubt. Legally, spiritually, emotionally, it remained as Turkish in Turkish eyes as the Golden Horn—even though it lay 20 miles (32 kilometers) inside Syria.

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In recent days bitter fighting between extremists from the Islamic State and Syrian Kurdish militias in the area has sent thousands of refugees streaming across the Turkish border. As IS fighters advanced on the town of Ayn al Arab (Kobane), near the tomb, Turkish leaders faced the question of what to do if Suleyman Shah's tomb is captured, let alone destroyed, as IS fighters have been known to destroy tombs and mosques elsewhere in Syria.

On Thursday Turkey's parliament voted 298 to 98 to authorize military action against Islamic State forces. It's unclear to what degree the tomb factored into the decision, but Turkish President Recep Tayyip Erdoğan has repeatedly said that his country would defend the tomb, which it regards as sovereign territory.