Article: Battle of Moscow
Section: Mozhaisk defense line (13 – 30 October)

Question: During the battle of Moscow what prevented the Germans from reaching Tula until October 1941?

Mozhaisk defense line (13 – 30 October).
By 13 October 1941, the "Wehrmacht" had reached the Mozhaisk defense line, a hastily constructed double set of fortifications protecting Moscow's western approaches that extended from Kalinin towards Volokolamsk and Kaluga.
Despite recent reinforcements, only around 90,000 Soviet soldiers manned this line - far too few to stem the German advance.
Given the limited resources available, Zhukov decided to concentrate his forces at four critical points: the 16th Army under Lieutenant General Rokossovsky guarded Volokolamsk, Mozhaisk was defended by 5th Army under Major General Govorov, the 43rd Army of Major General Golubev defended Maloyaroslavets, and the 49th Army under Lieutenant General Zakharkin protected Kaluga.
The entire Soviet Western Front—almost completely destroyed after its encirclement near Vyazma—was being recreated almost from scratch.
Moscow itself was also hastily fortified.
According to Zhukov, 250,000 women and teenagers worked building trenches and anti-tank moats around Moscow, moving almost three million cubic meters of earth with no mechanical help.
Moscow's factories were hastily converted to military tasks: one automobile factory was turned into a submachine gun armory, a clock factory manufactured mine detonators, the chocolate factory shifted to food production for the front, and automobile repair stations worked fixing damaged tanks and military vehicles.
Despite these preparations, the capital was seemingly within striking distance of German "panzers" and was targeted in massive air raids, although these raids caused only limited damage because of extensive anti-aircraft defenses and effective civilian fire brigades.
On 13 October 1941 (15 October 1941, according to other sources), the "Wehrmacht" resumed its offensive.
At first, the Germans attempted to bypass Russian defenses by pushing northeast towards the weakly protected city of Kalinin, and south towards Kaluga and Tula, capturing all except Tula by 14 October.
Encouraged by these initial successes, the Germans launched a frontal assault against the fortified line, taking Mozhaisk and Maloyaroslavets on 18 October, Naro-Fominsk on 21 October, and Volokolamsk on 27 October after intense fighting.
Because of the increasing danger of flanking attacks, Zhukov was forced to fall back, withdrawing his forces east of the Nara River.
In the south, the Second Panzer Army initially advanced towards Tula with relative ease because the Mozhaisk defense line did not extend that far south and no significant concentrations of Soviet troops blocked their advance.
However bad weather, fuel problems, and damaged roads and bridges eventually slowed the Germans, and Guderian did not reach the outskirts of Tula until 26 October 1941.
The German plan initially called for a rapid capture of Tula followed by a pincer move around Moscow.
The first attack, however, was repelled by the 50th Army and civilian volunteers on October 29 after a desperate fight that occurred within sight of the city.
On 31 October, the "OKH" ordered a halt to all offensive operations until increasingly severe logistical problems were resolved and the "rasputitsa" subsided.