How did the USA end World War II?
There were two main theaters of war in World War II. There was the European theater, where the war ended in May of 1945 and the Pacific theater where it ended in August of that year.
In the European theater, the US did not end the war alone by any means. The war ended when the Allies (the US and England and some others from the west and the Soviets from the East) invaded Germany and took Berlin. Hitler committed suicide and the Germans surrendered.
In the Pacific, the US did end the war alone. It did this by strangling Japan and then using atomic bombs to force Japanese surrender. The US had pushed through the Pacific to the doorstep of the main Japanese islands. They had cut off almost all shipping to Japan. They were bombing Japan mercilessly. By doing all these things and then dropping atomic bombs, the US ended WWII.