There has been an enormous amount of international intervention in civil wars since 1945 that some have argued served to extend wars. According to Patrick M. Regan in his book "Civil Wars and Foreign Powers" (2000) about 2/3rds of the 138 intrastate conflicts between the end of World War II and 2000 saw international intervention, with the United States intervening in 35 of these conflicts. While intervention has been practiced since the international system has existed, its nature changed substantially. It became common for both the state and opposition group to receive foreign support, allowing wars to continue well past the point when domestic resources had been exhausted. Superpowers, such as the European great powers, had always felt no compunction in intervening in civil wars that affected their interests, while distant regional powers such as the United States could declare the interventionist Monroe Doctrine of 1821 for events in its Central American "backyard". However, the large population of weak states after 1945 allowed intervention by former colonial powers, regional powers and neighboring states who themselves often had scarce resources.