The National Post has a great article about the US and UN. Here are a few quotes…
In the history of the United Nations, only one country has ever asked the world’s permission to go to war. That country is the United States. ... No other country has shown the same deference. Of the two dozen or so other major international conflicts to occur since the UN’s founding, not one was carried out under the Security Council’s authority. Russia did not ask permission to invade Hungary, or Czechoslovakia, or Afghanistan. China did not ask permission to invade Tibet. India did not ask permission to invade Pakistan, and Pakistan did not ask permission to invade India. Syria did not ask permission to invade Lebanon. France and Britain did not ask permission to invade Egypt in the Suez crisis, and none of the Arab countries ever asked permission to attack Israel, whether in 1948, 1956, 1967 or 1973. ... The UN has neither proved sufficient to prevent war, where it has occurred, nor necessary, where it has not. This is no accident. It is a paradox woven from its own premises. International law is very good at preventing the outbreak of war between, say, Italy and Norway: democratic countries that observe the rule of law in their own affairs and are disposed to be bound by it abroad. But these are the very sorts of country that are least likely to go to war with one another in the first place. On the other hand, the countries that are most likely to make war—dictatorships, particularly rogue states—are the least likely to be constrained by the niceties of international law.
via Ipse Dixit