Stuka
The Stuka was a dive-bomber airplane used on Earth, by Germany, during World War II. Their standard equipment included sirens used to frighten the enemy during an attack, and an armament of either 7.92 millimeter machine guns, 20 millimeter cannons, or larger, anti-armor, 37 millimeter cannons.

In 1944, when  was trapped in an alternate timeline, Vosk dispatched a squadron of Stukas refitted with plasma cannons, one of them carrying the registry "A5 CH", to destroy Enterprise in order to prevent it from in turn destroying the temporal conduit the Na'kuhl were constructing. ()The Stuka was also used in the mirror universe on Earth during that planet's 20th century''. (, opening credits)

Appendices

Background information 
The historical "Stuka" was an acronym for "Sturzkampfflugzeug", German for dive-bomber airplane, and endowed with the official designation  "Junkers Ju 87". () With their for the times pinpoint accuracy and wailing sirens they are nowadays often considered in countless recent documentaries and treatises on the subject, the original, what is presently called,  "" weaponry for their effectiveness as such during the 1939-1940 Blitzkrieg campaigns in Europe in World War II. The Stukas seen in the opening credits of "In a Mirror, Darkly" were actually featured as such in footage taken from an actual, contemporary German propaganda reel. According to the accompanying German contemporary audio commentary these Stukas were attacking the French  during the May-June 1940 Blitzkrieg campaign in France. 

The "A5 CH" tail code seen on one of the Ju 87s belonged to a real Ju 87 R-2, seen in historical photographs; the Stukas featured in the episode are also the R-2 variant, albeit retrofitted with plasma cannons.

The "Storm Front" sequence on the other hand, including the Stukas, was an entirely CGI created effect, done by Fred Pienkos at Eden FX. ("Visual Effects Magic", ENT Season 4 DVD special features)

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