The Allied build-up continued, more aircraft became available and new airfields in eastern Algeria and Tunisia were built. The Allies reduced the flow of Axis troops and equipment into Tunis and Bizerta, but a sizable Axis force was already ashore. On January 23, 1943, the Allied Eighth Army took Tripoli, Erwin Rommel's main supply base. Rommel had anticipated this, switching his line of supply to Tunis with the goal of blocking the southern approach to Tunisia from Tripoli at Gabès. The Mareth Line, which the French had built to protect against an Italian attack from Libya, was  Allied troops had already crossed the Atlas Mountains and set up a forward base at Faïd, in the foothills on the eastern arm of the mountains, an excellent position to thrust east to the coast, split the Axis forces in southern Tunisia from the forces further north, and cut the line of supply to Tunis. Elements of the 5th Panzer Army, headed by General Hans-Jürgen von Arnim, reached the Allied positions on the eastern foot of the Atlas Mountains on January 30. The 21st Panzer Division met French troops at Faïd, and, despite excellent use of the French guns, which caused heavy casualties among the German infantry, the defenders were easily forced back. U.S. artillery and tanks of the 1st Armored Division then entered the battle, destroying some enemy tanks and forcing the remainder into what appeared to be a headlong retreat. This was, however, a trap, and when the 1st Armored Division gave chase it was engaged by a screen of German anti-tank guns, and sustained heavy casualties. A U.S. forward artillery observer whose radio and landlines had been cut by shellfire recalled,  The 21st Panzer Division resumed its advance towards Faïd. American infantry casualties were exacerbated by the practice of digging shallow shell scrapes instead of foxholes, as German tank drivers could easily crush a man inside a scrape by driving into it and simultaneously making a half-turn. Several attempts were made by the 1st Armored Division to stop the German advance, but all three combat commands found that each defensive position they tried to occupy had already been overrun, and they were attacked by German troops with heavy losses. On 2 February, the 1st Armored Division was ordered to end its attacks and concentrate to form a reserve. The Germans captured most of Tunisia, and the entrances into the coastal lowlands were blocked. The Allies held the interior of the roughly triangular Atlas range, but with the exits blocked this was of little advantage to the Allies.