To what extent does the War on Terrorism represent a break with previous United States foreign and defense policy?

To some extent it is a break with previous policy, but not completely.

In general in the past, we have not fought wars to prevent things from happening.  We have not gone out and invaded places so that they wouldn't harm us later.  However, there are exceptions and the main one there is Vietnam.  There, we went and fought in a country because we feared that communism would be a danger in the future if we did not fight it there.

In general, in the past, we have not done things like tracking people that we do not like and trying to kill them with missiles (from the drones).  But I think this is as much because we have new technology as anything.  I think that in the past we would probably have used such technology if we had it.

So I do not really think that the War on Terror is really morally or philosophically different from the Cold War.  It's just a bit more complicated and has "better" technology.