What is "great" but a label we slap onto this or that person or event or thing? The king may consider himself "great" for winning the war, as may his queen, his family members, and just about anyone. Or then just about anyone even himself may not consider himself to be "great" for winning the war. In the end the King can be guaranteed to be able to consider himself "great" but he may have to radically reframe his view on what "greatness" means to himself in order to achieve this.
Winning a war may make a king greatly known but the qualities that make a king great are much more complex and do not necessarily involve any war at all. A great king is loved and respected by his people. This quality can help a king win a war but so can a king whose people fear him, and a king who is feared by his own people is not that great.