"Nothing get done that isn't sanctioned" - so if the "command HQ" far away is destroyed everyone packs up and goes home for tea? Assuming it happens like that is a bit like imagining that somewhere there is an Internet Central Office, and if that ceased to operate the whole Internet would shut off.

The Irish Republic was never "a safe operational base" for the IRA, in the sense that they could operate there in the open or without difficulty. It's fairly clear that arms dumps were sited, and training carried on in Northern Ireland, or for that matter in England, when that was felt to be appropriate.

For terrorist activities in the USA it seems pretty plausible that the best place to stock (and purchase) arms, and to carry on training, would be in North America itself, rather than in some far off country. The value of "a safe operational base" would seem to be primarily for morale purposes, as a place for "rest and recuperation", and it could be dispensed with if necessary.