Today, a security clearance from the Department of Defense still earns you a "Visitor, Escort Required" badge from the Department of Homeland Security. Or most intelligence agencies. The reverse is also true. The reasons why aren't important. The organizational histories are reasonable, there are no villians.
However, a systemic view of national security quickly points up the folly of the current patchwork regime, with its redundancy and lack of organizational trust. If we need to share information quickly across the system of national security, then it is time to consider the behaviors that are nothing more than dysfunctional at the system level.
The DoD Information Sharing Strategy speaks of sharing information with unexpected partners, driven by unanticipated events. Perhaps it is time to reconsider also the list of expected partners, due to events that are becoming increasingly anticipated.