By the 1900s, the US was emerging as a world power, and no longer a regional power. By this stage it was looking beyond its hemisphere - no longer at small acquisitions on the continent, but at its security in the Atlantic and in its interests in the Pacific and East Asia. In both of these areas, its geopolitical interests were similar to the UK - to combat the rise of the German navy, to prevent a German hegemony in Europe, and to keep China open to all the powers for trade. This is in addition to the UK being the largest foreign investor in the US, and its close economic and trading relationship. It is difficult to have the US go to war against the UK in the 20th century without a radical POD going back decades earlier. Not to mention the costs of going to war against the greatest world power made the benefits negligible.