In my belief America is merely uptaking the role of the World's leading nation; you'll find that while Britain, even France were on top, they began or took part in many wars in order to secure their intrests or safeguard world peace(which is usually a good excuse to secure your own agenda), acting as a 'global police force' simply because they had the means to do so.
America i would argue only became a 'World power' of significance after WW1 and then eventually a 'Superpower' after WW2. Only then do we see a gradual and more dominant role in Global conflicts, firstly as part of a UN coalition in Korea, and then after the transformation of the affluent 1950's, becoming more assertive and aggressive, involving itself in South East Asia, the Middle east, Africa and even South and Central America. Of course the Cold War was a vast influence on most of the interventions, but so too was commercial intrests, namely oil. The US uses its status as Britain before it did to secure materials that will, inevitably, make it money.
Sylday is right, America does give huge aid, in fact the Bush admin gives more to Africa than the Clinton admin ever did; but still lets not forget that along with aid is a seemigly endless supply of armaments which exacerbates despitism. This is not to say America is alone in such acts, Britain is in fact the second largest exporter in the world of weapons, after the USA. - the line of arguement goes, " that if these peoples who want weapons are just going to get them off criminal organisations or other nations; why not sell them ourselves and make a proffit." So to say that America, and indeed Britain have the betterment of the worlds intrests at the forefront of their mind is ludacris; its all about the security of commercial intrests and gaining money. In this Britain, from it repressive imperial history, is by far, more guilty than America is.