Complex sentence: In the summer of 1944, work began to enlarge and reinforce many of the Wolf's Lair original buildings, however the work was never completed because of the rapid advance of the Red Army during the Baltic Offensive in autumn 1944.
Split into simpler sentences: In the summer of 1944, work began to enlarge and reinforce many of the Wolf's Lair original buildings. However, the work was never completed because of the rapid advance of the Red Army during the Baltic Offensive in autumn 1944.