The Fuso Sea Incident is an event which took place in 1937 within the Strike Witches timeline. This was the first battle where Striker Units were widely employed in combat, although Neuroi sightings had occurred earlier (mostly land-type units).
In 1937, shortly after the invention of Striker Units, some number of Neuroi suddenly appeared above the Fuso Sea. Although there was no hive present, it still was a considerable threat to Fuso, therefore a Army-Navy joint fighter wing, as well as a naval fleet, was dispatched to counter it.
This incident made many Fuso Witches famous, especially Anabuki Tomoko, who starred in a propaganda movie highlighting her achievements in the incident, Flash in the Fuso Sea. Though it was actually Katou Keiko who scored the highest kills in the battle, she was not featured as prominently. This was because the Fuso Army top brass favored dogfighting, and felt Keiko's fighting style, which was mostly based on mid-ranged shooting and hit-and-away tactics, was unsuitable for a movie.
This incident was the first time Neuroi started taking machine-like forms instead of previous organic forms. Therefore, it can be considered the first battle of the Second Neuroi War.
At first, the Fuso military forces did not encounter much trouble, though the situation quickly changed when the Neuroi switched over to faster, more durable drones. Coupled with a logistics disaster where the various factions' equipment lacked standardization and were incompatible with each other; as well as many Witches burning out from exhausting their magic, only the coming of winter and hurried operational reorganization allowed Fuso to withstand the battle.
With the coming of spring, the Neuroi launched a major offensive on both air and land, depleting the military's forces even further. The teishin (close range raiding) operation was employed in response. The operation's success of eventually led to the Neuroi withdrawing, though by then, the losses the Army Air Service Witch Force had been too severe for it to go on without major recruiting. Flash of the Fuso Sea was made to fulfill this purpose, and it indeed raised the profile of both the military and Witches in general within Fuso -- Inagaki Mami and Sakomizu Haruka are two examples of Witches who started their military careers after being inspired by the movie.