Since our inception, the U.S. had waged numerous campaigns without declaring war. In 1812, 1846, 1898, 1917 and 1941 the U.S. Congress declared war. I left out the U.S. Civil war 1861-1865.
The last declared war by the U.S. Congress was in 1941. Since then, the U.S. has waged several large "police actions". The most recent was the ‘authority to use force’ in 2002 that the U.S. Congress gave President Bush. In other words, the permission to wage war against Iraq.
The wars fought 1950-1953 (Korea) and 1965-1972 (Vietnam) were undeclared wars like today. The threats then were thought to be grave enough to wage open warfare, yet too dangerous to declare war against a government. This applies today. If the U.S. had declared war in 2002 against Iraq, it was deemed possible that a number of Arab governments would act likewise to defend their colleague. I still find this unlikely, although the price of getting it wrong would be very steep.
Looking at it another way, the "authority to use force" was a way for the U.S. Congress to get out of the long-term difficulty of having made any mistakes. If all went well, it could take credit for being ‘courageous’. If the war went bad or turned out to be a mistake, it could just take it all back. That sure is a nice position to be in, and we all know how political figures like to pay for their mistakes. The problem that I have is that time is NOT on our side. WMD makes it imperative that we wage offensive warfare to prevent future attacks.
The known nuclear powers in the world are:
8) North Korea
South Africa had developed nuclear weapons, but has since been in compliance with dismantling nuclear weapon systems.
Israel most likely has nuclear weapons.
If an attack with nuclear weapons occurs anywhere in the world, the aggressors will most likely have come from the Islamic world. The material and/or weapons may have actually been built elsewhere, but the cause of the strikes will be in the name of Islam. This actually significantly raises the risk of WMD being used. Once nuclear weapons are used, openly declared war against governments would become likely.