That is complicated. Most of the documents with the details seem to be Resticted or Classified. There is some procedure for the ATC to request military interve

That is complicated. Most of the documents with the details seem to be Resticted or Classified. There is some procedure for the ATC to request military intervention. This process at least used to be slow complciated, and requiring approvals at the highest levels. (Think office of the Secretary of Defense levels). That was pre 9/11 though. I've not come across any good information since then. Separately, the military can act on its own initiative against potential threats. This is so much simpler that I'm pretty sure it is what gets used in practice. For example, if a plane is heading into DC now, without appropriate permission, the military will initiate the intercept on its own (this particular scenario is documented publicly, but with limited details, obviously). There is no restriction on ATC providing relevant information in such situations. It is even legal for ATC to alert the military of the situation. The alert teams used for these fall under NORAD authority. Lastly, what happens even more often than that, is that the ATC will ask nearby military planes if it is possible for them to come and take a look at something. Depending on their mission, it is sometimes possible to the military plane to do so. This is nothing out of the ordinary, and ATC can make such requests of any plane, but most airliners are likely to be in to big a hurry for scheduling reasons.