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Commander-in-Chief

When negotiations with the Soviet Union failed in 1939, and aware of the imminent war and deploring the lack of equipment and preparation of the army, Mannerheim resigned once again from the military council on 17 October 1939, declaring that he would agree to return to business only as Commander-in-Chief of the Finnish Army. He officially became the supreme commander of the armies, at the age of 72, after the Soviet attack, the November 30, 1939. In a letter to his daughter Sophie, he stated, "I had not wanted to undertake the responsibility of commander-in-chief, as my age and my health entitled me, but I had to yield to appeals from the President of the Republic and the government, and now for the fourth time I am at war."

He addressed the first of his often controversial orders of the day to the Defence Forces on the day the war began:

The President of the Republic has appointed me on 30 November 1939 as Commander-in-Chief of the armed forces of the country. Brave soldiers of Finland! I enter on this task at a time when our hereditary enemy is once again attacking our country. Confidence in one's commander is the first condition for success. You know me and I know you and know that everyone in the ranks is ready to do his duty even to death. This war is nothing other than the continuation and final act of our War of Independence. We are fighting for our homes, our faith, and our country.

The defensive field fortifications they manned became known as the Mannerheim Line.

Field Marshal Mannerheim quickly organised his headquarters in Mikkeli. His chief of staff was Lieutenant General Aksel Airo, while his close friend, General Rudolf Walden, was sent as a representative of the headquarters to the cabinet from 3 December 1939 until 27 March 1940, after which he became defence minister.

Mannerheim spent most of the Winter War and Continuation War in his Mikkeli headquarters but made many visits to the front. Between the wars, he remained commander-in-chief. Although Mannerheim's main task was to lead the war, he also knew how to strengthen and maintain the will of the soldiers to fight. He was famed for this quote:

Forts, cannons and foreign aid will not help unless every man himself knows that he is the guard of his country.

Mannerheim kept relations with Adolf Hitler 's government as formal as possible. Mannerheim did not really appreciate Hitler, even though he initially expressed an interest in his rise to power; his attitude towards Hitler turned negative at the point when Mannerheim's visit to Germany made him realize what kind of "ideal state" Hitler was building; he compared Hitler's rise in Germany to the 1917 rise of the Bolshevists in Russia. Before the Continuation War, the Germans offered Mannerheim command over 80,000 German troops in Finland. Mannerheim declined so as to not tie himself and Finland to Nazi war aims; Mannerheim was ready for cooperation and fraternity with Hitler's Germany, but for practical rather than ideological reasons because of the Soviet threat. In July 1941 the Finnish Army of Karelia was strengthened by the German 163rd Infantry Division. They retook the Finnish territories annexed by the Soviet Union after the Winter War, and went further, occupying East Karelia. Finnish troops took part in the Siege of Leningrad, which lasted 872 days.

On 20 June 1942, Mannerheim was inspecting Finnish front line troops in Poventsa when he and his staff were engaged by a Soviet anti-tank gun with direct fire. One shell exploded near Mannerheim who, on varying accounts, took cover either in a trench or behind tree trunks until Finnish counter-battery fire neutralized the attacker. Captain Ahti Petramaa, section commander during the inspection, lost fingers from flying shrapnel and was escorted to an aid station where he was promoted to major by Mannerheim.