From an early age Gamelin showed potential as a soldier, growing up in a generation seeking revenge on Germany for the annexation of Alsace-Lorraine at the end of the Franco-Prussian War.

When Gamelin came back to Paris in 1897, he entered the prestigious École Supérieure de Guerre and finished second of his class of about eighty of the best future officers in the French Army.

Charles Lanrezac, then second-in-command of the École Supérieure de Guerre, and later a general in the early days of World War I, noted Gamelin as an intelligent, cultivated, and industrious young officer, bound to earn higher functions in the future.

In the region of Noyon, he showed sophisticated tactical skills by gaining ground without losing lives needlessly (which had been atypical earlier in the war, see Attaque à outrance).

[8] The defensive stance implied by the construction of the Maginot line contradicted with the promises of an offensive into western Germany should any of France's Eastern European allies be attacked by the Reich.

[9] The Abyssinia crisis of 1935-1936 shattered Gamelin's hopes for a Franco-Italian alliance as Britain insisted on having the League of Nations impose sanctions on Italy.

[14] Gamelin's policy of keeping the military out of politics was especially appreciated by the politicians in Paris as his predecessor, Maxime Weygand, a fiery right-wing Catholic had spoken on matters well outside of his purview.

At his first meeting with Blum on 10 June 1936 at the Hôtel Matignon, Gamelin expressed his willingness to serve the new government while also arguing for greater defense spending.

To move troops from the Maghreb in turn required command of the western Mediterranean Sea, which could be challenged by the Regia Marina of Fascist Italy, whose foreign policy was becoming increasingly pro-German and anti-French.

[30] The water-logged, low-lying lands between Cassel and Dunkerque would have been extremely expensive to extend the Maginot line into and likewise the same was true of the heavily industrialised area around Lille.

[32] In his plans for a "forward defense", Gamelin expected the main German blow to come on the flat, open plains of Belgium and saw the Ardennes forest as an area of secondary importance.

[36] Led by the Secretary-General at the Quai d'Orsay, Alexis St. Leger, a number of French diplomats in direct defiance of Bonnet wanted to create an alliance designed to deter Germany from war that would consist of France, Great Britain, Poland, the Soviet Union, Romania, Yugoslavia, Greece and Turkey.

[39] Gamelin wrote to Daladier on 3 December 1938 that main issue was : "whether France wishes to remain a great power, indeed whether or not she wants to succumb sooner or later to the Nazi yoke.

[39] To win against the Reich, Gamelin argued that he needed Britain to make what the British called the "continental commitment" (i.e. sent a large expeditionary force to France).

[41] The War Secretary, Leslie Hore-Belisha, tended to equate quantity with quality, and failed to understand that the long years of neglect caused by the limited liability doctrine left the British Army short of equipment and trainers for the proposed 33 division force.

[42] Much of the French guerre de longue durée ("war of the long duration") was based on the assumption that France would have a strong Eastern European ally, preferably the Soviet Union, to divert German forces east.

[43] After the German-Soviet non-aggression pact on 23 August 1939 shattered that assumption, and Gamelin fell back to an alternative eastern strategy of reviving the Salonika front.

[44] Gamelin gave orders for the Armée du Levant based in Syria to prepare to embark from Beirut to Thessaloniki on 25 August 1939 to open up an alternative eastern front in the Balkans.

France saw little action during the Phoney War, apart from a few French divisions crossing the German border in the Saar Offensive, who advanced a mere 8 km (5.0 mi).

A major problem for Gamelin was the weakness of the Armée de l'air, most of whose fighter aircraft was known to be obsolete, which left France very exposed to Luftwaffe bombing attacks.

Gamelin was initially keen to open up a second front in the Balkans, and made extensive preparations to move the Armée du Levant to Greece in the first weeks of September 1939.

[45] However, at a meeting at the Supreme Allied Council on 22 September 1939, the French plan for a revived Salonika front met with fierce opposition from the British.

[51] Daladier, who very wanted to execute Weygand's plans for a new Salonika front, complained in December 1939 that Gamelin's advice "resembled sand running through your fingers".

[53] Gamelin was forced to abandon plans to raise an additional 13 divisions owing to manpower shortages as he could not conscript so many men without crippling French war industry.

[54] Admiral Darlan in particular started to press for France to take action, saying la guerre de longue durée strategy was not working.

[54] In the corridors of power, Darlan argued that France should under the pretext of aiding Finland in the Winter War cut Germany off from its supply of high-grade Swedish iron ore.[55] Gamelin provided an opening about asking for a study from Darlan about how best to aid Finland, and instead received a memo saying that France needed to strike before Germany signed another economic agreement with the Soviet Union by cutting Germany from the Swedish iron ore.[55] Darlan called for a joint Army-Navy expedition to seize the Petsamo province of Finland recently occupied by the Red Army even though it would almost mean war with the Soviet Union out of the hope of provoking German response, which would allow French forces to occupy northern Sweden, and hence deprive the Reich of its most important source of iron, which was the crucial element needed to manufacture steel.

Gamelin favoured an aggressive advance northward to meet the attacking German forces in Belgium and the Netherlands, as far removed from French territory as possible.

The speed of this advance, German air supremacy, and the inability of the British and French to counter-attack undermined the overall Allied position to such a degree that Britain abandoned the conflict on the continent.

The Vichy regime tried Gamelin for treason along with other important political and military figures of the Third Republic (Édouard Daladier, Guy La Chambre, Léon Blum, and Robert Jacomet) in the Riom Trial.

Imprisoned by the Vichy regime in Fort du Portalet in the Pyrenees, he was later deported by the Germans to the Itter Castle in North Tyrol with a few other French high officials.