Heather Hurlburt wrote for New York magazine about historical precedents for presidents sharing classified material:
In Trump’s case it seems likely that he disclosed sensitive information about ISIS out of incompetence. But there’s a precedent for presidents selectively revealing government secrets when it suits them. Our last septuagenarian president, Ronald Reagan, made a great display of giving the nation classified information, related to the existence of a military-grade airfield in Grenada, in the run-up to the 1983 invasion of the Caribbean island.
Presidents are allowed to do this, because they exercise supreme executive power. When Trump had no access to classified information and the power it brings, he was an incessant scold on the importance of strict rules of confidentiality, and punishment for even minor violations of data-handling protocol (the principle that gave us “Lock her up!”). It’s not really surprising that, having discovered the power, both perceived and actual, that flows from government secrets, this president would seek to use that power aggressively — and be more than a little careless in the process.