Complex sentence: The first IR devices were experimented with in the pre-WWII era, but did not become practical as a missile guidance system until the introduction of conical scanning and miniaturized vacuum tube were developed during the war.
Split into simpler sentences: The first IR devices were experimented with in the pre-WWII era. Truly practical designs did not become practical as a missile guidance system until the introduction of conical scanning and miniaturized vacuum tubes during the war.