Messerschmitt Bf.110C, German users, part one

The Messerschmitt Bf.110, often mistakenly known as the Me.110, was a twin-engine heavy fighter (Zerstörer) anf figther-bomber developed in Germany during the late 1930s.
It was armed with two 20mm MG FF cannons, four 7.92mm MG 17 machine guns and one 7.92mm MG 15 machine gun, or, in late variants, twin-barreled MG 81Z for defensive purposes.
Development work on an improved type, the Messerschmitt Me.210 started before the beginning of the war, but its aerodynamical problems hindered it considerably, pushing the Bf.110's operational history until the end of the war alongside its replacements, the Me.210 or the Me.410 (which was a much improved version of the Me.210).
The Bf.110 enjoyed a considerable success early in the war, during Polish and Norwegian campaigns and during the Battle of France. Its main weakness was its lack of agility once in the air, but it could be mitigated with the correct tactics. That weakness was oftenly exploited by the Royal Air Force during the Battle of Britain, when the Bf.100 was flying as a close escort for German bombers.
When the Royal Air Force began to target German soil with night bomber raids, some Zerstörer units were redeployed and re-trained as night fighters, role which the aircraft proved to be well suited for. In other theatres of operations, like North Africa, Mediterranean and Eastern front, it acted successfuly as an air-superiority fighter and strike aircraft. It also defended German soil against raids of USAAF's 8th Air Force, until they changed tactics making the Bf.110 increasingly vulnerable to American aerial supremacy during 1944.
During Balkans and North African campaigns, as well as in the Eastern Front, it provided valuable close air-support as a potent fighter-bomber and later in the war it acted as a formidable radar-equipped night-fighter which became the main night-fighter of the Luftwaffe. Most of the German night fighter aces, flew the Bf.110 at some point in their careers, and the top night ace of all time, Major Heinz-Wolfgang Schnaufer flew it exclusively and claimed 121 victories in 164 combat missions.

Sources:
1. https://en.wikipedia.org/wiki/Messerschmitt_Bf_110
2. Salamander Books - The Complete Book of Fighters