I went to Cottonwood Cove, took out all the legion lads there, set Mrs Withers (?) free, then went to the fort and powered up Mr House's Securitons (for better or worse, I don't know yet). Then I went and took out Caesar and his lieutenants or bootlickers (can you tell how much I despise them?) in his fancy tent. So now ED-E, Veronica, and I are back heading to Red Rock Canyon to contact the Khans. Suddenly, here comes another bunch of those Legion yahoos to tell me that Caesar's not pleased with me. They've been re-spawning for days now. Don't they stop once I've taken out the Big Man himself?

No, Legate Lanius takes command of the Legion after Caeser dies.

Hmm. So I didn't take him out? Where's he holed up?

Even if you take out the leadership of the Legion, they will still send assassins after you. In my case daily, but I think some weird timescale trickery may be in effect there.

Playing hit squad Whack-A-Mole is a daily occurrence - on PC there's http://www.nexusmods.com/newvegas/mods/45303/? that changes the Legion & NCR frequency.

At the conclusion of the game.