What act set the tone for the westward expansion of the united states of america by establishing that new states would be formed and admitted to the united states, instead of allowing existing states to expand their territory?

The Monroe Doctrine adopted in 1823 was the key moment of the big migrations to the west of what is now the western part of the USA. This act put European nations on notice that the US would defend other nations of the Western Hemisphere by further colonization. So people had felt secure enough to move westwards and that they will not be under the aggression of any other country. Also the biggest portion of the Indian tribes were already put under control, as well as the government measure to give people free land when they move in those parts, made it very attractive for people to start migrating westwards.