What happened on lexington green on april 19, 1775?

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On April 19, 1775, the American Revolution fired shots toward Lexington in Massachusetts. This concluded in the Battles of Lexington and Concord. British troops demolished battle stores in Concord were  fought by militiamen at Lexington. 

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Imperialism is a policy or ideology of extending a country's rule over foreign nations, often by military force or by gaining political and economic control of other areas.

For short:

a policy of extending a country's power and influence through colonization, use of military force, or other means.