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I'm looking for a short story about a two factions engaged in a space war where scientists of one faction figured out a better weapon, but they decided not to waste effort producing it, because, by the end of designing that weapon, they figured out they can actually build an even better weapon. But, once they finished designing that second weapon, they figured they can make an even better one. This repeated until they came to the ultimate weapon, but by which time the other faction already defeated them using the plain old weapons from the beginning of the war.
The story is told from a perspective of an officer of the losing faction explaining why they lost.