A major operation is a large-scale undertaking, usually involving a significant amount of resources and personnel. The two crossword solutions for the term 'major operation' are 'DRILL' and 'WAR'. A DRILL is a military exercise that is conducted to practice and improve combat readiness. WAR is an armed conflict between two or more nations or states. Both drills and wars are considered major operations due to their complexity and the resources required to carry them out.