Because they can be used for weapons and tanks.
Bantu, The bantu tribe migrated across Africa bringing iron with theim in arround 3000 BC.
In the Mexican-American War, Mexico faced an enemy that was coming into its own as a military power. In March 1836, Mexican forces overran the Alamo in San Antonio, Texas, achieving victory over those who had declared Texas independence from Mexico just a few weeks earlier
Manifest Destiny, a phrase coined in 1845, expressed the philosophy that drove 19th-century U.S. territorial expansion. Manifest Destiny held that the United States was destined—by God, its advocates believed—to expand its dominion and spread democracy and capitalism across the entire North American continent.
And here is a good link describing/ sumamrizing it: https://www.history.com/topics/westward-expansion/manifest-destiny
