 The Germans had to break the line to capture Thessaloniki , Greece 's second @-@ largest city , with a strategically @-@ important port . The attack started on 6 April with one infantry unit and two divisions of the XVIII Mountain Corps . Due to strong resistance , the first day of the attack yielded little progress in breaking the line . A German report at the end of the first day described how the German 5th Mountain Division " was repulsed in the Rupel Pass despite strongest air support and sustained considerable casualties " . Two German battalions managed to get within 600 ft ( 180 m ) of Fort Rupel on 6 April , but were practically destroyed . Of the 24 forts that made up the Metaxas Line , only two had fallen and then only after they had been destroyed . In the following days , the Germans pummelled the forts with artillery and dive bombers and reinforced the 125th Infantry Regiment . A 7 @,@ 000 ft ( 2 @,@ 100 m ) high snow @-@ covered mountainous passage considered inaccessible by the Greeks was crossed by the 6th Mountain Division , which reached the rail line to Thessaloniki on the evening of 7 April . 
