Close support of artillery and combat engineers was an important factor in the success of Operation Uranus. However, with rare exceptions, all Soviet guns and howitzers at this time were towed rather than self-propelled. This lack of mobility proved to be greatly exacerbated by the absence of roads, the presence of deep snow cover and a scarcity of artillery tractors. Towed guns were also highly vulnerable to counterattack while on the move, especially since they were often hauled by horses or their own crews. The 152 mm heavy howitzers were particularly difficult to maneuver; owing to their great weight, they were incapable of crossing rivers on anything but tank bridges and were prone to becoming hopelessly mired and needing to be abandoned by their crews. This situation did not satisfy the state authorities. In November 1942 the State Defense Committee ordered the development of a heavy self-propelled gun armed with the 152.4mm ML-20 howitzer.