In my opinion it was in the middle of the 19th century, during the administration of the 11th president of the United States, James K. Polk.
After the original 13 states became independent, it was very clear that the new nation wanted more land. The first acquisition, the Louisiana Purchase of 1803 by Thomas Jefferson, was peaceful and it doubled the territory of the United States.
In 1821 Mexico won its independence from Spain. The United States immediately started coveting their lands. In the end it would steal them by force of arms. What is today Texas, New Mexico, Utah, Nevada, Arizona, California and part of Colorado was Mexican sovereign territory.
In the night of the inauguration of James K. Polk, a Democrat, [The Democratic Party has always been the party of war], he confided to the Secretary of the Navy that one of his main objectives was the acquisition of California.
In 1845 the Washington Union, a newspaper expressing the position of President Polk and the Democratic Party, had spoken clearly:
“Let the great measure of the [Texas] annexation be accomplished, and with it the question of boundaries and claims. For who can arrest the torrent that will pour onward to the West? The road to California will be open to us. A corps of properly organized volunteers... will invade, overrun, and occupy Mexico. They would enable us not only take California, but to keep it. ”
It was shortly after that, in the summer of 1845, that John O'Sullivan, editor of the Democratic Review, used the phrase that became famous, saying it was “Our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.”
In other words, it was the same argument used by the Nazis in the 20th century: “We are a master race, guided by God.”
James K. Polk started the first Yankee imperialist war against Mexico.
President William McKinley, started the second imperialist war, the war against Spain, by ordering the blowing up of the Maine in Havana harbor.
Barack Obama and his mercenary Zionist Israelis will start the next war against Iran.
Historical record from Wikepedia and “A People's History of the United States 1492 – Present” by Howard Zinn.