The War of 1812 - it's wrong to assume that the USA was fighting on its own as the greater part of the Royal Navy and nearly all the British Army was keeping an eye on a certain Mr Bonaparte and his navy and army for most of the time. The only lesson that the Royal Navy learnt was that British ships could not always take on bigger ships and win, as they had often done in the past.

The answer so far seems to be that seldom does a nation fight a war on its own - as it makes much more sense to get other nations to help out. Start by making a list of wars that are fought by single nations and work on from there.