People who were in the room, IOW, witnesses, reported it. The CIA pulled a spy out because of it.
"Trump “revealed more information to the Russian ambassador than we have shared with our own allies.”"
"The comments, which have not been previously reported, were part of a now-infamous meeting with Russian Foreign Minister Sergei Lavrov and Russian Ambassador Sergey Kislyak, in which Trump revealed highly classified information that exposed a source of intelligence on the Islamic State. He also said during the meeting that firing FBI Director James B. Comey the previous day had relieved “great pressure” on him."
Truth does have a liberal bias.
You cling to Trump just as he clings to Putin.
Who's interests are furthered by Trump's latest actions in Syria and pulling out of the Open Skies Agreement?
America's or Russia's.
(Not to mention believing Putin over our intel agencies.)