In 1844, James Polk ran for president on a platform of "manifest destiny". This was a vision and ambition of many Americans at the time that as a superior people in character and religion, it was our God given right that our country should span from sea to shinning sea. That was red meat to the religious Americans, but spelled big trouble for the indigenous peoples that occupied the land that Polk was eyeing, in some cases as long back as 12,000 years. At the time, the border of Texas stopped at the Nueces River, not down at the Rio Grande where it's at today. The federal government made an offer to the Mexican government to purchase the land between the Nueces and Rio Grande.
When that offer was rejected, Polk dispatched Union soldiers to build a small fort on the north bank of the Rio Grande. Seeing this, Mexican regulars attacked the fort. President Polk told the American people that the Mexican army crossed into Texas and attacked a US military installation. And this began the Mexican American War, and the beginning of Polk's plan to push America to the pacific. Many members of congress at the time wanted the border to be moved south all the way to Mexico City, and that threat was used to force Mexican surrender and the acceptance of the Treaty of Hidalgo. In which, by use of force, the US relieved Mexico of its land and increased the size of the US by 1/3 adding New Mexico, Arizona, Nevada and California to the union. Today, politicians love to point at Putin and criticise and demonize him for doing far less.
We have a long history of imperialism while pointing fingers at other imperialistic countries. Again, Ron Paul understood this, Gary Johnson understands this.