The information the president relayed had been provided by a U.S. partner through an intelligence-sharing arrangement considered so sensitive that details have been withheld from allies and tightly restricted even within the U.S. government, officials said.
The partner had not given the United States permission to share the material with Russia, and officials said Trump’s decision to do so endangers cooperation from an ally that has access to the inner workings of the Islamic State. After Trump’s meeting, senior White House officials took steps to contain the damage, placing calls to the CIA and the National Security Agency.
https://www.washingtonpost.com/world/national-security/trump-revealed-highly-classified-information-to-russian-foreign-minister-and-ambassador/2017/05/15/530c172a-3960-11e7-9e48-c4f199710b69_story.html?utm_term=.d98b8a87db2f
I just read this earlier today. This may be the most serious problem Trump has had so far. I'm not sure he knows that he can't say whatever he wants to just anyone.