Bannon is said to be "the biggest obstruction" to McMaster doing his job. In one recent situation, Secretary of Defense James Mattis, Secretary of State Rex Tillerson and McMaster had agreed that they would provide a range of options for the President in terms of how to proceed in Afghanistan, including a modest increase in the number of US troops in Afghanistan.
But before McMaster could make the recommendation, the option of a troop buildup was leaked to the media, with part of the leak including that Bannon opposed such a move. The blowback from conservatives and others caused the recommendations to be delayed, and others who supported the move to get "skittish." The leak was perceived within the NSC as Bannon or his allies trying to stamp McMaster's name onto the proposal and to push back against it through the media.
An NSC spokesman disputed that version of events to CNN, saying that the strategy was still under review as principals address certain concerns. "It had nothing to do with the leak," the spokesman said.
