We managed to have access to untapped resources that gave us commerce and trading as we expand more land within North America. Industrial Revolution and practices in agriculture swooped through across the U.S. in the 19th Century, attracting mass immigration from Europe, Asia, and Latin America in the 19th and 20th centuries which helped fuel the economy and industrial power that we gained at the beginning of the 20th Century. By the end of the Civil War, the United States had the most powerful military within the North American sphere and something that impressed the European powers. Our energetic and surprising ideas like Light Bulb, automobiles, first-powered flight aircraft, famous sodas, products like Kodak, etc provided our need that helped compete with most of the world powers. During WWII, we produced millions of war materials that helped our struggling Allies against Germany and Japan out of reach of enemy bombing. The war obliterated and bankrupted the age of imperial world empires and into a new emerging world based on democracy and free ideals. Meanwhile, much of this country was left unscathed by the war (aside from bombing of Pearl Harbor and Aleutian Islands Campaign), allowing the flow of materials to help rebuild war-torn Europe and Asia, generating more money into our economy.
The most important development was the atomic bomb, which was produced in the U.S. with the assistance of British and Canadian scientists during WWII. The use of two nuclear weapons against Japan made the U.S. made us realized the importance of the weapon and how powerful we became. It helped spearhead the establishment of the U.S. Air Force (separated from the Army previously known as the Army Air Forces) to carry out strategic bombing and nuclear war. The U.S. Navy was more powerful thanks to the necessity of aircraft carriers meaning we can fly and bomb targets without having to capture land to launch air attacks, as well as the U.S. Army and U.S. Marine Corps understanding the role of ground warfare in changing doctrines and tactics.