I have to say if we can bring back the WWII japanese imperialist bastards vs the Al Queda bastards it will be a tough call as to who is more brutal.
However, no contest as to who will win. The japanese were efficient as well as merciless.
A few years ago, Spike TV had a series which pitted historical 'badass' groups against each other. "Deadliest Warrior." One example was the IRA vs the Taliban (IRA victorious.) Another was Japanese Samurai vs Vikings. Pretty interesting stuff.