During the occupation, Friesland, Groningen and Drenthe were on the front line of the air war. Many Allied air routes to Germany lay over these provinces. Especially during bombing raids on northern German cities, there was a coming and going of hundreds of heavy bombers. Initially, this happens only at night, but from 1943 onwards also during the day.

For protection, the occupying forces build a formidable defence belt. Cities and important military targets are secured by anti-aircraft guns. In Friesland, anti-aircraft artillery was positioned on the Wadden Islands, the coastal strip of Het Bildt, in Harlingen, Lemmer, Franeker and Gaasterland and in the area around Leeuwarden airfield.

German observation posts give directions to the Flieger Abwehr Kanone (Flak) to knock Allied aircraft out of the sky. Radar probe stations can detect enemy aircraft as far away as 120 to 150 kilometres. The German radar positions in Friesland are 'Schlei' (Tench) on Schiermonnikoog, 'Tiger' (Tiger) on Terschelling and 'Eisbär' (Polar Bear) near Sondel in Gaasterland.

Flak alone, however, proved insufficient to intercept the Allied bombers. In the first two years of the war, night-fighter units were stationed at eight Dutch airfields, including Leeuwarden. From here, German pilots shoot down hundreds of Allied aircraft. Most end up in Friesland, Groningen, North Drenthe, in the IJsselmeer and in the Wadden area.

The air war caused many casualties. In Friesland, it is estimated that around 450 Allied aircraft and around 150 German fighters crashed during the war years.

In Wolvega's general cemetery, Allied honorary graves remind residents of the seven fallen crew members of the Lancaster JB 545 that crashed in the fields on the Sas (below Sonnega) on 16 December 1943. The bomber was on its way to Berlin and was shot out of the sky by German fighters here.

Near the crash site, the "Monument to Allied Airmen" was unveiled by the sister of one of the crew members in 2004 as a tribute to the fallen crew. A boulder with a plaque and a panel with background information on what happened keep the memory alive.  

Of seven soldiers killed, five were British, one Canadian and one Australian. They were buried side by side. Identical white natural stone tombstones with an embossed emblem and cross have been placed at each grave. The cross is not only a symbol of the Christian faith, but also a reminder of the sacrifice made by war victims for a life of freedom.