In 1939 he was a reserve lieutenant. Because of his language skills he was sent to Oflag IV-AHohnstein as a translator. The POWs at Hohnstein were mostly French officers including 28 generals and an additional seven Dutch and 27 Polish generals. Eggers felt his experiences at this camp were a very poor preparation for his time at Colditz. POWs and guards treated each other with respect and there were no real problems or friction.
On 22 November 1940,Eggers received his orders to report to Oflag IV-C, Colditz. He started as LO3 (Lager offizier 3 or duty officer) and was faced with rebellious, anti-German POWs from Poland, France and the UK, who took every opportunity to harass their captors. Later these were joined by Belgian, Dutch and American officers.
Eggers tended to treat his opponents as difficult schoolboys and always tried to retain his calm and dignity even when provoked to the utmost. On one occasion his cap was stolen by a POW (to be measured and copied for an escape). He calmly waited for a guard to get a new one before he left the building. British and Dutch officers agreed that Eggers always treated them correctly. Lieutenant Damiaen J. van Doorninck, a former Dutch POW, wrote in his foreword for Eggers's book that:
This man was our opponent, but nevertheless he earned our respect by his correct attitude, self-control and total lack of rancour despite all the harassment we gave him.
On 1 June 1941, Eggers was promoted to Hauptmann. In February 1944, he became the Security Officer for the camp, a post he retained until the camp was liberated by the U.S. Army in April 1945.
After the liberation of Colditz Castle by the U.S. Army, Eggers retired from active service and returned to his post as a teacher in Halle, Saxony-Anhalt, as he was able to prove that he had never joined the Nazi Party. He became a headmaster, and then a lecturer at Halle University. In September 1946, as he was in the Soviet Occupied Zone, he was arrested by the Russians and questioned about Gestapo agents in Colditz. Charged with crimes against humanity, spying and supporting a fascist regime, he was sentenced to ten years labour. This was spent inSachsenhausen and then Torgau Prison.