Prior to the commencement of World War Two in September 1939 the Swiss Air Force was mostly equipped with obsolete aircraft including the hopelessly outdated Dewoitine D-27 fighter, a 1927 designed and poorly armed parasol monoplane of which 66 were produced in Switzerland from 1931 (it was armed with just two 7.5mm machine guns). This was until January 1939 when they received 10 Messerschmitt Bf 109D-1 Dora fighters (fitted with a two-bladed propeller, Junkers Jumo 210D engine and armed with two wing-mounted and two nose-mounted 7.92 mm MG 17 machine guns) from Germany to help train pilots for the arrival of 80 Bf 109E-3 Emil fighters.
The first Messerschmitt Bf 109E-3 Emil fighters (fitted with an uprated Daimler-Benz DB 601Aa engine, tri-bladed propeller and increased armament including the standard two 7.92 mm MG 17 machine guns above the engine and new 20mm MG FF wing cannons) were delivered from Germany between June 1939 and April 1940 (30 were originally purchased and delivered in June 1939, 50 more were delivered between October 1939 and April 1940). So by the serious outbreak of war in Western in 1940 the Swiss, although outnumbered could at least provide an adequate defence of their airspace with a modern fighter aircraft.
A beautifully restored Bf 109E is on display today at the Swiss Air Force Centre in Dübendorf (Flieger Flab Museum). This was also once the base for the first Bf 109E’s of Flight Company 21 of the Swiss Air Force in 1939. In 1940 they were used to intercept and in some cases bring down German Luftwaffe aircraft such as the Heinkel He 111 bomber and Messerschmitt Bf 110 heavy fighter that crossed the Swiss border.
There were 6,501 instances of border violation by German and Allied aircraft in Swiss airspace during World War Two; 244 foreign aircraft landed (forced or with issues), crashed or were shot down (including several Luftwaffe aircraft in 1940) over Swiss territory; 1,620 aircrew were interned. 3 Swiss aircraft were shot down by German aircraft in 1940, the Swiss high command then decreed that only anti-aircraft guns would be used to engage foreign aircraft, which lasted until October 1943 when air operations commenced again. In September 1944 another Swiss fighter was shot down by a USAAF aircraft.
Whilst Allied and German aircraft entered Swiss airspace, it seems large Swiss markings were applied to the underside of the wings of the Bf 109 more for the benefit of Swiss anti-aircraft gunners on the ground so as to not mistake them for intruders. Later most Swiss aircraft received red and white neutrality stripes on the wings and fuselage to help with their national identification (see the historical Bf-109 photos above, they were not trying to sneak up on anyone, they were there to noticeably defend Swiss airspace!).