Complex sentence: Friessner, fearing that his forces might be enveloped by Malinovsky's 2nd Ukrainian Front and by the 4th Ukrainian Front, flew to Hitler's headquarters to request permission to withdraw his forces to defensive positions along the Tisza River.
Split into simpler sentences: Friessner, fearing that his forces might be enveloped by Malinovsky's 2nd Ukrainian Front and by the 4th Ukrainian Front, flew to Hitler's headquarters. He requested permission to withdraw his forces to defensive positions along the Tisza River.