Simply because we use our head does not make war disapear. We where able to end the cold war without a shot fired and there have been numerous treaties and such created in the last 50 years that have gone a long way to preventing unneccessary wars. However, even with this climate of anti-war sentiment in the world body and with organizations such as the UN to prevent them, how many wars have been fought across the globe in the past 50 years? A large number.
In fact, modern history is filled with examples of how fearing war has lead to even more war. Of course the prime example is WW2 where everyone kept trying to cut deals with Nazi Germany but this strategy ultimately only lead to an extremely disasterous conflict for the continent.
Simply acting responsibly does not prevent wars. In fact, many time actions that are at the time claimed to be 'responsible' are later seen as reprehensible in the historical perspective. Case in point is the Rawandan genocide in which the world body sat back, intellectualized, discussed, and debated while 800,000 humans where slaughtered. Declaring war on the Hutus for their barbaric murder of the Tutsis could have saved hundreds of thousands of innocent men, woman, and children, but instead we decided to think about it and do nothing.