FreeCol:Colopedia.concepts.interventionForce.description/en

Any attempt of your colonies to declare independence will inevitably weaken your home country. This is in the interest of your home country's enemies. For this reason, one of them will dispatch an Intervention Force to support you against the Royal Expeditionary Force in the War of Independence, provided you can produce a sufficient number of liberty bells. Over the course of the game, both the Intervention Force and the Royal Expeditionary Force will be reinforced with further units.