Late Saturday evening President Trump lashed out at allies Britain, France, and Germany via twitter related to the United States' Syria withdrawal. He urged European nations to take responsibility for their own captured foreign fighters in Syria by repatriating and prosecuting them; otherwise, Trump warned, the terrorists could "permeate Europe" upon their release.

"The Caliphate is ready to fall," Trump tweeted. "The alternative is not a good one in that we will be forced to release them..." He said that the US "does not want to watch" Islamic State (or ISIS) "permeate Europe," which he indicated would be the inevitable outcome.

The United States is asking Britain, France, Germany and other European allies to take back over 800 ISIS fighters that we captured in Syria and put them on trial. The Caliphate is ready to fall. The alternative is not a good one in that we will be forced to release them........ — Donald J. Trump (@realDonaldTrump) February 17, 2019

As part of the strange threat and sure to be controversial statements, the president further reaffirmed total US victory over ISIS as he mentioned US forces would imminently begin "pulling back after 100% Caliphate victory."

Earlier this month the Wall Street Journal cited US defense officials to indicate that "the military plans to pull a significant portion of its forces out by mid-March, with a full withdrawal coming by the end of April."

....The U.S. does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go. We do so much, and spend so much - Time for others to step up and do the job that they are so capable of doing. We are pulling back after 100% Caliphate victory! — Donald J. Trump (@realDonaldTrump) February 17, 2019

The fate of the US-backed Syrian Democratic Forces (SDF), mostly Syrian Kurds, has remained a huge unknown as Turkey has been amassing troops along its border to invade in the aftermath of a US draw down. Some Pentagon generals have also warned of the rapid comeback of ISIS in the wake of any "power vacuum".

Trump's latest tweet warning that ISIS foreign fighters would "permeate Europe" is also likely a reference to the fact that the SDF still maintains many hundreds of captured ISIS terrorists in its jails. SDF leadership has echoed a similar warning of late, saying it'll be forced to release the jihadists as a result of any rapid US exit.

The UN has estimated that in total up to 42,000 foreign fighters traveled to Iraq and Syria to join IS — which appears a very conservative estimate — and which includes about 900 from Germany and 850 from Britain.

Identified as French nationals fighting within ISIS' ranks, via Khaama press news agency

SDF leaders have previously complained about the "lack the capacity" for mass incarceration of ISIS terrorists and the inability to have proper battlefield trials for them. Recent estimates have put the number of ISIS militants in US-SDF battlefield jails at almost 1000, though Trump put the number at 800 in his tweet.

However, even once they do return to Europe it's unclear the extent to which they'll be properly prosecuted and locked in prison by European authorities.

For example, fresh controversy has lately erupted in Britain after a 29-year old UK woman traveled to join ISIS and was convicted for membership in a terrorist group upon her return to Britain. She was jailed but is now walking free merely less than three years after her conviction.