President Donald Trump on Wednesday continued to defend his decision to withdraw US troops from northeastern Syria, abandoning Kurdish forces in the region, by saying the Kurds did not help the US during World War II.

"They didn't help us in the Second World War; they didn't help us with Normandy," Trump said of the Kurds, who played a vital role in the US-led campaign against ISIS.

And when asked by reporters whether he felt the Syria retreat and treatment of the Kurds sent a poor message to other potential US allies, Trump said, "Alliances are very easy."

Trump's comments came hours after Turkey launched a military operation against the formerly US-backed Kurdish forces in Syria.

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President Donald Trump on Wednesday defended his decision to abandon the Kurds to a Turkish military incursion in Syria by saying they didn't help the US during World War II.

This came amid reports that Turkish ground troops were crossing the border into Syria after air strikes that began earlier in the day.

"They didn't help us in the Second World War; they didn't help us with Normandy," Trump said of the Kurds. He added, "With all of that being said, we like the Kurds." The Kurdistan region, which stretches across the borders of Iraq, Syria, Turkey, Iran, and Armenia, is roughly 4,000 kilometres (2,500 miles) from Normandy, France. The Kurds are a distinct ethnic group — with an estimated population between 25 to 35 million — that have never had a permanent or official nation state.

Earlier on Wednesday, Trump said in a statement released by the White House that he did not endorse the Turkish military operation and thought it was a "bad idea." But he did not refer directly to the Kurds or signal any immediate response from the US to thwart Turkey's actions.

Read more: Turkey launches military operation against the Kurds in Syria just days after Trump abandoned them

The Trump administration on Sunday abruptly announced the US was withdrawing troops stationed in northeastern Syria ahead of a Turkish operation.

The move has been broadly condemned in Washington, including by top congressional Republicans and former Trump administration officials, as many feel Trump paved the way for Turkey to go after key US allies.

The Kurdish-led Syrian Democratic Forces (SDF) bore the brunt of the US-led campaign against ISIS, losing about 11,000 fighters in the process.

Ahead of the Trump administration's announcement, Kurdish forces had recently dismantled defensive positions along the Turkey-Syria border under assurances from the US it would not allow a Turkish assault. The SDF described Trump's decision to withdraw troops as a "stab in the back" and made clear it felt betrayed by the US.

'Alliances are very easy'

Shortly after his reference to WWII on Wednesday, when he was asked by reporters whether he felt the Syria retreat and treatment of the Kurds sent a poor message to other potential US allies, Trump said, "Alliances are very easy." The president said it "won't be" hard for the US to form new partnerships.

Trump also said "our alliances" have "taken tremendous advantage of us."

But a number of congressional lawmakers and former US officials have expressed concerns about the message sent to allies or future partners by the Trump administration's Syria retreat.

Read more: Republicans and former US officials rip into Trump for abandoning the Kurds in Syria

Trump's former top envoy in the fight against ISIS, Brett McGurk, was particularly critical of the president.

McGurk in a tweet on Monday said, "Donald Trump is not a Commander-in-Chief. He makes impulsive decisions with no knowledge or deliberation. He sends military personnel into harm's way with no backing. He blusters and then leaves our allies exposed when adversaries call his bluff or he confronts a hard phone call."

Similarly, Republican Sen. Lindsey Graham of South Carolina, a key ally for Trump in Congress who sits on the Senate Foreign Relations Committee, on Monday tweeted, "By abandoning the Kurds we have sent the most dangerous signal possible — America is an unreliable ally and it's just a matter of time before China, Russia, Iran, and North Korea act out in dangerous ways."

Graham on Wednesday announced he had reached a bipartisan agreement with Democratic Sen. Chris Van Hollen of Maryland for "severe sanctions" against Turkey in light of the operation against the Kurds. "While the Administration refuses to act against Turkey, I expect strong bipartisan support," Graham said in a tweet.

In a separate tweet, he added, "America is better than this. Please stand up to Turkey, Mr. President.

Turkey is a fellow NATO member but has a complicated relationship with the US. Turkish President Recep Tayyip Erdogan's critics view him as a dangerous autocrat and enemy to democracy.

There are also fears that the Turkish operation will create a security vacuum and open the door to the resurgence of ISIS while also serving to the benefit of Syrian President Bashar Assad, Iran, and Russia. Kurdish forces have been detaining thousands of ISIS fighters, and many fear the Turkish operation will pave the way for their escape. Trump on Wednesday said if the ISIS fighters were to get out, they would be "escaping to Europe."

Along these lines, many US lawmakers have questioned the logic of Trump's decision and how it benefits US national-security and strategic interests.

'This attack will spill the blood of thousands of innocent civilians'

The Kurds and Turkey have been at odds for years, and the dominant fighting force in the SDF — the Syrian Kurdish People's Protection Units (YPG) — is viewed by the Turkish government as a terrorist affiliate because of its ties to the Kurdistan Workers' Party (PKK). The PKK for decades has waged a violent campaign against the Turkish government as part of a broader effort to establish an independent Kurdish state within Turkey.

In tweets announcing the onset of Turkey's military incursion into Syria on Wednesday, Erdogan said the operation's goal was to "neutralize terror threats against Turkey and lead to the establishment of a safe zone, facilitating the return of Syrian refugees to their homes."

Read more: Trump's Syria retreat is a massive break from post-9/11 Republicanism

"Our mission is to prevent the creation of a terror corridor across our southern border and to bring peace to the area," Erdogan said.

Meanwhile, the SDF has pleaded with the US and its allies to establish a no-fly zone in the region and "carry out their responsibilities to avoid a possible impending humanitarian disaster."

"This attack will spill the blood of thousands of innocent civilians because our border areas are overcrowded," the SDF's official Twitter account said.