Two options are available to the players in such a scenario. To gain power and to survive, countries can attack and capture other areas ruthlessly as Napoleon and Hitler did. The other option is to see that a balance of power is maintained. This can be done by diplomatic relations, alliances or by force. Bilateral trade agreements, aid to other countries are examples of diplomatic relations. NATO and the European Union are examples of alliances. There was a time when the US and the USSR were the two balancing powers of the world. The US attacked and also supplied arms to Vietnam fearing that the spread of communism in the country and other places may shift the balance of power in favour of the USSR. It would be effective to define realism at this point. A Cambridge University Press publication defines realism in three steps. An instance where this theory does not hold good or becomes ineffective is the case of terrorism. Terrorism is practically invisible even if countries who abet terrorists are identified. The terrorists do not come to the negotiating table nor do they work in visible organized structures. If the opponent is not available for negotiations or to fight with, it is not possible to think of survival or strength. A country may be hit by a terrorist attack at any time. Security becomes practically ineffective if suicide attacks are resorted to by the terrorists. Even a strong nation like the US can be shocked by attacks such as the September 11 suicide attacks on the Twin Towers. But there is practically nothing that can be effectively done by any country in such a situation. The US attacked Iraq and set up a new administration.