When the Fokker D.VII appeared on the Western Front in April 1918, Allied pilots at first underestimated the new fighter because of its squarish, ungainly appearance, but quickly revised their view. The D.VII's unique ability to seemingly "hang on its propeller," and fire into the unprotected underside of enemy aircraft made it a highly feared combat opponent. The Armistice agreement requirement specifically demanding that all Fokker D.VIIs be immediately surrendered attested to the general high regard for the airplane.