Syrian Kurdish forces, until this week the stalwart of the U.S. battle against Islamic State, have begun re-honing their skills as guerrillas after President Donald Trump suddenly gave the green light for a Turkish incursion into Syria.

Some Kurds had only just returned from a 45-day campaign fighting ISIS in eastern Syria when Trump’s announcement came, according to Patrick Cockburn, a veteran reporter for the Independent. He spoke with Rojvan, a Kurdish commander, who recently buried one of his men killed by an ISIS rocket.

“We acted like a regular army when we were fighting Daesh (ISIS),” said Rojvan, who serves in the People’s Protection Units (YPG), Cockburn reported on Wednesday. “But now it is we who may be under Turkish air attack and we will have to behave more like guerrillas.”

Trump told Turkish President Recep Tayyip Erdoğan that his military could enter Syria during a phone call on Sunday, backing off from commitments to the Kurds to protect them from a unilateral Turkish invasion. Turkey labels the YPG as terrorists and indistinguishable from the Kurdistan Workers’ Party (PKK), which has battled for autonomy for more than three decades inside Turkey at the cost of 40,000 lives. The PKK is recognised as a terrorist group by the United States and European Union.

Rojvan said the Kurds were now at a big disadvantage militarily.

“We are mainly armed with light weapons like the Kalashnikov and the RPG (rocket propelled grenade) launcher and light machine guns, but we will be resisting tanks and aircraft,” he said.

Cockburn said that while Trump, faced with a deluge of criticism, had stepped back a little from his vague agreement with Erdoğan, threatening Turkey with economic sanctions should it go to far, an invasion will be difficult to prevent now the momentum has started.

“We are already on the down-slope leading to the ethnic cleansing of up to 2 million Kurds in the vast triangle of land which the Kurds call Rojava in northeast Syria,” Cockburn said.

“Much of the Kurdish population lives in cities and towns like Qamishli, Kobani and Tel Abyad just south of the Syrian-Turkish frontier. They are unlikely to wait to see what a Turkish occupation, backed by bands of Syrian Arab paramilitaries with links to al Qaeda type groups, is like.”

The brazenness of Trump’s betrayal of the Kurds is breathtaking, he said.

“All the credit for defeating ISIS is given to U.S. forces under Trump’s wise leadership, while in reality the U.S. role was almost entirely confined to air strikes and artillery fire,” he said.