To elaborate on why the job security is so good, it's because of the clearance system. During the internship, my manager said that if I wanted to come back next summer I would have to get top-secret (TS) clearance. Almost everyone working there has at least secret (S) or TS clearance. Getting clearance takes ~1 year WITH an employer backing you, and it may take years (a.k.a impossible) without one. So once you're in the system with clearance, you can bounce around the def-sec world forever.