A simplified definition of imperialism is when a strong country takes over a weaker one. Imperialism was used by Europe and Japan during the 1800s to 1900s.
Before beginning imperialism, a country had to go through industrialization. The country would then look for more raw materials, and they generally had to go out to a new place to find them.
One of the best examples of imperialism is the Roman Empire. Alexander the Great was a perfect example of a nationalist who pushed imperialism forward. With his massive army, he conquered most of Europe. He also named at least 8 cities after himself.
England was also a big contributor to the spread of imperialism, as well as the White Man's Burden. The White Man's Burden was an attitude of superiority toward the native cultures of the invaded countries. Through wars and treaties, the British expanded their empire to include most of Europe, some of Africa, and the Americas.
The Babylonian Empire was enormous around 600 BC. The empire spanned most of Western Asia. King Nebuchadnezzar II created the Hanging Gardens to ensure his wife was happy in the deserts of Babylon. The Hanging Gardens are now considered one of the Seven Wonders of the World.
I know we all like to think of the U.S. as perfect, but the truth is, we did do an act of imperialism. When the U.S. stumbled upon the Hawaiian islands, who had their own monarchy and everything, we realized that we didn't have any island states (Rhode Island didn't count, apparently). So, without consulting the native aristocracy, we took over and named Hawaii the 50th state.