Turkish tanks rolled across the border into Syria on Wednesday. Protected by warplanes and flanked by special forces, they quickly succeeded in forcing Islamic State militants out of the city of Jarablus and driving a wedge between their territory and that of the Syrian Kurds.

President Recep Tayyip Erdoğan said Turkish-backed rebels — mostly Arab and Turkmen — had taken control of the city.

Why this incursion?

While directed at the self-proclaimed Islamic State, the operation, dubbed “Euphrates Shield”, is also about stopping Kurdish militants from filling the void left by the fanatical Sunni group.

Turkey has been rocked by a series of bombings in recent months for which the Islamic State claimed responsibility. But its overriding strategic concern is still the risk that Kurdish nationalism in Syria will fan the flames of Turkey’s own Kurdish insurgency.

The Turkish Kurdistan Workers’ Party (PKK) last year returned to armed struggle after peace talks with Erdoğan’s government collapsed.

It has ties with the Syrian-based People’s Protection Units (YPG), a largely Kurdish organization which controls the border with Turkey east of the Euphrates River.

Syrian Kurds also control three self-governing cantons in the northwest of Syria, called Rojava. Turkey’s fear for years has been that the two might connect. “We cannot let this happen,” the defense minister, Fikri Işık, said this week.

Why now?

Planning for the incursion may go back years. There were reports last summer that a Turkish intervention was imminent.

Erdoğan said a suicide bombing this weekend in Gaziantep, Turkey’s seventh largest city, convinced him to act.

“We have said ‘enough is enough’ and there needs to be a putting an end to this,” he said on Wednesday.

More than 300 Turks have died in terror attacks in the last twelve months, the single largest being the dual bombing of a peace rally in Ankara in October 2015.

The intervention also comes at a time when Western-backed rebels are fighting intensely with forces loyal to President Bashar al-Assad in and around Aleppo, Syria’s largest city before the uprising. Russia, Assad’s protector, has stepped up air and missile strikes. Turkey itself is just emerging from a botched military coup for which thousands of officers and officials have been purged. Any and all of these factors may have contributed to its decision to act now.