From FoOlRulez:

Hiraga Saito was summoned to the wonderful world of Halkeginia through the use of magic. He was summoned by the very cute, yet with "Zero" magical ability, Louise. Although it was an accident, Saito was summoned during the summoning ceremony, and sealed the agreement to become Louise's "Familiar" through a kiss. That is how he arrived in this world.

While on the way to visit the Princess of Tristain for her upcoming marriage to the King of Germania, war broke out. Because of the upcoming marriage, Conquista joined forces with Albion, declared war and began their invasion of Tristain. While drowning in despair over the disparity in military strength, Louise, by chance, got on board the Zero Fighter piloted by Saito. Her ability to use the legendary magic lineage "Void" awoke and she single-handedly ensured the triumph of Tristain over the army of Albion. And so we continue on from there to their next scene of battle.