Trump’s latest intervention in the showdown between Qatar and the Saudi-led bloc last week led to a new round of recriminations between Riyadh and Doha:

Mr. Trump’s hand may be weakened by his own insistence on taking sides. After initially supporting the Saudis, he appeared to take a more balanced approach at the urging of Mr. Tillerson, who knows leaders on both sides from his previous career as an oil executive.

Qatar is home to the largest American air base in the Middle East and is the center of military operations against the Islamic State in Syria and Iraq.

But at the White House on Thursday, Mr. Trump appeared to take a new jab at Qatar [bold mine-DL] when he complained of the “massive funding of terrorism by certain countries.”

He added, “If they don’t stop the funding of terrorism, I don’t want them to come together.”

No president can be a credible mediator between feuding parties when he has frequently and publicly taken sides against one of them. Whatever chance the U.S. might have had in resolving this dispute quickly was lost months ago when Trump first aligned himself with the Saudis and their allies in his public remarks. That had the effect of undermining anything Tillerson tried to do to forge a compromise, and it reassured the Saudi-led bloc that they would face no consequences from Washington because the president was with them. Despite Trump’s recent statements that he wants to be a mediator, he has already proven that he favors the Saudi-led bloc, and that makes the bloc’s members much less likely to agree to any compromise that might be arranged. Trump’s claim last week that there would be “a deal worked out very quickly” shows how oblivious he is to this.

Like other Saudi-led misadventures in the region, the attempt to pressure Qatar into submitting to the bloc’s demands to change its foreign policy has backfired. If the Saudis were hoping to force Qatar to take a harder line with Iran, for instance, they could not have miscalculated more. Just last month, Qatar and Iran reestablished full diplomatic relations:

Qatar said Thursday that it has restored diplomatic relations with Iran, marking a further break with Arab nations that have closed ranks against Qatar for its links to Islamist groups and others perceived as regional threats.

The decision ignores demands by Qatar’s neighbors — led by Saudi Arabia — to limit ties with Tehran and threatens to deepen the region’s worst diplomatic crisis in decades, which has complicated Washington’s policies in the Middle East.

The U.S. shouldn’t have taken any side in this dispute, and by siding with the Saudis and their allies Trump also managed to take the side that has shown the greatest ineptitude. On top of that, he invited this headache by going to Riyadh and for all intents and purposes giving the Saudis and their allies a green light to do anything they wanted. None of this has advanced American interests in the least, but it has shown our regional clients how easily they can manipulate and use Trump’s support for their own ends.