In August 1917, the British Royal Flying Corps (RFC) introduced the coordinated mass use of single-seat fighter aircraft for low-level ground-attack operations in support of the Third Battle of Ypres, with Airco DH.5s, which were unsuitable for high-altitude combat, specialising in this role. The tactic proved effective and was repeated at the Battle of Cambrai in November 1917 by DH.5s and Sopwith Camels being used in strafing attacks. While the tactic proved successful, losses of the unarmoured fighters proved to be extremely high, reaching up to 30 per cent per day. Most losses were due to ground fire, although low-flying aircraft also proved vulnerable to attacks from above by enemy fighters. Two-seat German fighters such as Halberstadt CL.II, originally designed as escort fighters, were also used ground-attack, playing an important role in the German counter-offensive at Cambrai. While the CL-type fighters were not armoured, the Germans also introduced more specialised heavily armoured two-seat aircraft such as the Junkers J.I for contact patrol and ground-attack work.