Ahaha, irony because the UK was notorious for being mostly impenetrable due to being isolated by sea. And as a result, they had a reputation for having the strongest navy/fleet.thaluikhain said:San Marino, unless you are Italy, an ally, or have conquered Italy.
Otherwise,US, Russia or Chinaanywhere that has a nuclear arsenal, preferrable a second strike capability, such as a SSBN fleet. Not going to happen.
Places like the UK would be shorter to invade, except that you can't, I guess.