I have always thought of them as a continuation of World Wars One and Two. The US has been expanding westward since the 1800s. After, through our "Manifest Destiny", we reached California and the Pacific Ocean, we next acquired Alaska and Hawaii and the Pacific Islands. We went even further west until we hit the Spanish Territory of the Philippines and Guam. We took those from Spain in 1898. At that point, we controlled most of the Pacific Rim. Only the Empire of Japan stood in our way as a regional threat. By 1945, we also had garrisoned Japan, eastern China, Samoa, Taiwan, and we were trying to control Southeast Asia. Our westward expansion was finally stopped dead in 1975 when we lost the Vietnam War. Now the focus is on moving eastward from our sphere of influence in Europe to control the Middle East. But since we are being stopped dead in Afghanistan now, our hopes of controlling Central Asia may never come to pass.