They provoke us, which causes us to retaliate which just provokes them further which leads to them further attacking us. It's just an infinitely recurring cycle until someone wins, since neither side is willing to give in. Hence why it breaks down once someone doesn't behave as the terrorists would like. And given the fact there's many more people in the UK, US armies etc. with far better equipment and resources, with advancing technology rates - If there's a winner, surely it's more likely to be us. So it's still not really 'successful'. I'd class a success as being Spain pulling out of Iraq after the Madrid bombings, the UK and US aren't going to give in to any terrorist threat soon, so it's just needless, unjustified irrational violence by religious extremists.