The domino theory is basically ,if one new country went communist in Asia then it would begin a chain reaction that would cause several more Southeast Asian countries becoming communist. it was the reason the united states went into the Vietnam war. since everyone was becoming a communist no laws were really set so they went into war. As the Vietnam War escalated, America's leaders and soldiers found themselves in a quagmire with no victory in sight. Eventually the war weakened the American economy, divided the people, and eroded the nation's morale.
In the Age of New Imperialism that began in the 1870s, European states established vast empires mainly in Africa, but also in Asia and the Middle East.