He divulged information that, with minimum effort, can be used to compromise the source of the information, resulting in not only the "burning" of that source but also a lowering of the confidence of US allies in sharing vital information with the US.
if you're a British / French / German / what-have-you intelligence agency, and you have sensitive information that will help a united effort to counteract terrorism, but you think the US president will carelessly blurt it out to Russian officials / NBC News / the cleaning lady, why would you share that information with the US?
Is everything you said here for a facy what happened? yes or no? Think carefully before or if you answer