Wars can be declared by countries other than super powers and these may eventually involve the world; new super powers may arise to challenge American superiority leading to new conflicts; a series of little wars all over the world may lead to the whole world getting involved and, finally, even small conflicts, like the present conflict caused by the terrorists can bring about a Third World War. If at all anything can prevent war, it would be if men are rational; but this is a pipe dream. All it requires is an ambitious politician to whip up emotions and rationality will fly out the window in an instant.
Firstly, it does not require superpowers to start wars. Both world wars were begun by countries, Germany and Japan, which were not strong enough to take on the whole world. They depended on taking their neighbors on by one. The Vietnam War did present some dangers of developing into a world war. All it requires is for a country to begin attacking its neighbor; soon there will be others taking sides and before long a full scale war would be raging. With many smaller countries, like Korea, now possessing nuclear weapons, it is not impossible for some simple border