Why did the pursuit of manifest destiny require wars or treaties?

The Mexican-American War was the first American military conflict fought entirely on foreign soil and the first to be closely chronicled by the press. The war ended with American victory and a treaty that increased the nation's size by more than half a million square miles. The subsequent diplomatic settlement of a controversy over the boundaries of the Oregon Territory added another quarter million square miles, creating a transcontinental nation-state. This rapid expansion of the nation's land area, coupled with dramatic military successes and developments in transportation (the railroad) and communication (the telegraph and the postal system), fueled theories of Anglo-Saxon supremacy that fused with national pride to produce Manifest Destiny, the conviction that white Americans were divinely ordained to dominate the continent, from sea to shining sea.

Manifest destiny was the belief that the US Bourders should be from the Atlantic to the placid if ocean. Other country's did not want to give up their land

Britain and its empire stood alone against hitler and mussolini. in june 1941, hitler broke the non-aggression agreement with stalin and germany invaded the soviet union. in december, japan attacked the us and britain. the main lines of world war ii had formed.