AskHistorians

Disclaimer: I'm not a historian, but thought this thread could use a more thoughtful answer than previous answers

Manifest destiny was mostly about extending the US from the Atlantic to the pacific, and trying excuse the genocide of indigenous peoples. Arizona and New Mexico were the last continental US states to receive statehood, both in 1912. World War 1 started in 1914 and the US joined the war in 1917. The war captured a lot of the attention of both politicians and everyday people, in large part because 5 million Americans directly joined the war effort, or close to 5% of the population. There were still territorial interests at play, but more about the strategic and resource value of the territories for war readiness vs settling them with Americans of European ancestry.