sentences:  During the World War II, the Slovak Air force was charged with the defense of Slovak airspace, and, after the invasion of Russia, provided air cover for Slovak forces fighting against the Soviet Union on the Eastern Front.While engaged on the Eastern Front, Slovakia’s obsolete biplanes were replaced with German combat aircraft, including the Messerschmitt Bf 109.The air force was sent back to Slovakia after combat fatigue and desertion had reduced the pilots' effectiveness.