The use of privateers by the Americans was the most effective action against British shipping. ..

The use of privateers by the Americans was the most effective action against British shipping. ..

Both "effective in" and "effective at" can be correct, depending on the context. Use "effective in" when referring to a specific area or situation where something is effective. Use "effective at" when referring to a specific task or action at which something is effective.

After the initial steps in resuscitation, a newborn is albeit with a heart rate less 100. What is the effective action to take in resuscitation

An effective resistance posture