Timoshenko at Summa — 11 February

After the Soviet disasters of December 1939 and January 1940 — Suomussalmi, Raate, Tolvajärvi, Kollaa — Stalin sacks the commander-in-chief Kliment Voroshilov on 7 January 1940 and entrusts the North-Western Front to Marshal Semyon Timoshenko, 45, a Khalkhin Gol veteran and one of the few modern Soviet generals to have survived the Great Purges.

In three weeks Timoshenko reshapes Soviet strategy from top to bottom. He masses his forces on the Karelian Isthmus along the Vyborg-Helsinki axis, the only theatre where a strategic victory remains possible; he adopts a doctrine of deep penetration backed by overwhelming artillery, 1,800 guns concentrated on 80 km of front; he adds massive air support of 1,000 Tupolev SB and Polikarpov aircraft. In all he gathers 600,000 men, 1,500 tanks — T-26s, BT-7s and the KV-1 prototype — and 2,400 guns. Facing him, on the Mannerheim Line, stand around 130,000 Finns, 50 anti-tank guns and no armoured reserve.

The offensive opens on 1 February 1940 with a nine-day artillery preparation, the most intense in military history to that date: more than 800,000 shells fall on the 80 km of front. The Summa sector, central bolt of the Mannerheim Line, is saturated. From 11 to 13 February the Soviet infantry, supported by KV-1 tanks that the Finns' 37 mm Bofors anti-tank guns cannot pierce, drives the defence in to a depth of five kilometres.

Timoshenko must decide how to exploit the breakthrough.