In some ways it is a break, and in others it is a continuation. It is a break from past foreign policy, at least in recent decades, in that we are nation building in Afghanistan and Iraq to try to combat future terrorism there. It is also a break in that our military, trained to fight conventional wars against clearly identifiable enemies, must now fight a diplomatic and military counterinsurgency. This required an adjustment.
It is a continuation in that domestic civil liberties were taken away, and it does closely resemble what we tried to do in the Vietnam War. In the 1980s and 1990s we tried to avoid these kind of entanglements because of the lessons of Vietnam, the expense and the time involved in successfully winning an abstract war.
One of the critical elements of this particular topic area is the "elusiveness" of the opponent. The War on Terror is uniquely different from other campaigns waged because of its setting. It does not involve traditional armies nor even traditional boundaries. It is one that is waged in cyberspace and across cellular devices and satellites. It is one that has been internalized as one that can exist within our own boundaries with terms like "sleeper cells" and "suspicious behavior" being openly posited and deployed in the general population. In this sense, the war on terror is one that demands new paradigms and solution in terms of fighting it. Yet, on some other levels, it is similar to other initiatives that strengthen the health of the state. In its early phases, the war on terror was predicated upon centralized authority guiding the citizenry and the idea that the people are "always under siege." In this light, voices of dissent are deferred and pushed aside because "the enemy is out there and wishes to do us harm." Like all wars, this helps to bring to light that individuals must follow the lead of their government who is supposedly driving out the terrorists in a state of war.
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.
I am not sure if there has been an explicitly stated change in United States foreign and defence policy, in view of greater perceived threat of terrorist activities directly on American soil. However it appears to me the since the 9/11 incident there is greater recognition among American policymakers, growth of terrorism directed against countries other than USA can promote terrorism against U.S.A. also. With this recognition there is greater willingness on part of U.S.A. to cooperate with other countries that have been bearing the brunt of terrorist activities.