Despite the length of the story, the allegations of substance are all in this single paragraph:
Trump went on to discuss aspects of the threat that the United States learned only through the espionage capabilities of a key partner. He did not reveal the specific intelligence-gathering method, but he described how the Islamic State was pursuing elements of a specific plot and how much harm such an attack could cause under varying circumstances. Most alarmingly, officials said, Trump revealed the city in the Islamic State’s territory where the U.S. intelligence partner detected the threat.
From that, WaPo argues:
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The identification of the location was seen as particularly problematic, officials said, because Russia could use that detail to help identify the U.S. ally or intelligence capability involved. Officials said the capability could be useful for other purposes, possibly providing intelligence on Russia’s presence in Syria. Moscow would be keenly interested in identifying that source and perhaps disrupting it.
Russia and the United States both regard the Islamic State as an enemy and share limited information about terrorist threats.