On April 6, 1941, Germany launched Operation Marita, the invasion of Greece, with the German 12th Army in Bulgaria launching offensives into southern Yugoslavia, whose capture would achieve the strategic objective of cutting off the rest of Yugoslavia in the north with Greece in the south. In the northern sector, units of the German XL Panzer Corps advanced along two points: the 9th Panzer Division for Kumanovo and the 73rd Infantry Division for Stip. Both met strong Yugoslav resistance, but they reached their objectives that same day. On April 7, the German 9th Panzer Division reached Skopje, and then Prilep the next day. On April 9, the Germans took Monastir. These thrusts cut the rail and road lines between Belgrade, Yugoslavia’s capital, and Salonika. More important, the Germans were now poised to invade Greece through the Monastir Gap.
Also on August 6, 1941, the Germans in western Bulgaria attacked from further south, with the German 2nd Panzer Division entering southern Yugoslavia and advancing to Strumica, which it captured that same day. After breaking off a Yugoslav counter-attack, on April 7, the Germans turned south for the border and, passing through the mountainous frontier, crossed into Greece and then overwhelmed the Greek 19th Motorized Infantry Division south of Lake Doiran. Now unopposed, the Germans continued south and entered Salonika on April 9, 1941.
(Taken from Invasion of Greece – Wars of the 20th Century – World War II in Europe: Vol. 6)
The German XVIII Mountain Corps crossed into Greece directly through the Metaxas Line. Here, strong Greek resistance, difficult terrain, and alpine weather conditions slowed the advance. But the Germans broke through at various points: the 6th Mountain Division through a 7,000-foot mountain that the Greeks had deemed impassable; the 5th Mountain Division at Neon Petritsi, and the 72nd Infantry Division from Nevrokop to Serres. Assaults on Greek fortifications along the Metaxas Line resulted in heavy casualties to the German 125th Infantry Regiment. The deeply entrenched Greek fortifications were finally subdued only after intense German air and artillery bombardment. Greek forces then surrendered to the Germans, who disarmed and released the Greek soldiers. East of the Metaxas Line, the German XXX Infantry Corps invaded Western Thrace, seizing Greece’s easternmost region by April 9. By then, all regions east of the Vardar River, including Salonika, Eastern Macedonia, and Western Thrace, were in German hands.