which BTW was an unfairly portrayed, but unfortunate statement. It is usual in a time of crisis, disaster, upset, for the senior military officer present - be he a General or a corporal - to state that I am in charge [unstated until relieved by a more senior person or the regularly appointed watch officer, duty officer] and this is the situation. It's how the military works. It is not an assumption of unauthorized authority. It is simply an acceptance of the responsibilities that fall upon the senior person present when things are going amiss.
One more thing for clarity, in the Haig comment he said I am in “control” here. His point was to tell the Ruskies there were not a coup in progress IMO as no one was sure what was happening in DC. Interesting that this could actually happen today given the climate in DC with the DS and anti Trumpers trying to wrest control of the government from the people’s choice.