Foreign intervention is support from other countries as you fight your War of Independence in Sid Meier's Colonization.
When you declare Independence from your mother country in Europe, foreign countries may offer to intervene on your behalf. Accumulating the number of liberty bells requested by a country will cause that country to join the war on your side. It will send reinforcements consisting of Continental Cavalry, Continental Armies, Artillery, and Men-O-War to some of your colonies. The intervening country will also bombard Tory cities with its navy, giving your units a 50% bonus when attacking them.