After attending classical high school in Voghera, he attended a cadet officer course in 1914-1915, graduating as artillery second lieutenant and participating in the First World War in the bombardiers, fighting on the Asiago plateau, on the Karst Plateau and in Gorizia during 1916 and 1917; he was promoted to lieutenant and later to captain, commanding the 423rd Bombard Battery during the assault on Monte Tonale in May 1918 and the 13th Bombard Battery that entered Trento in early November, after the battle of Vittorio Veneto.

He was replaced in this post by Niccolò Nicchiarelli on 19 April 1943 and returned to Italy, where after the fall of the Fascist regime on 25 July he was arrested in his estate in Santa Giuletta by the new government in August.

According to him, the National Republican Army (military of the Italian Social Republic) should have been born as an extension of the MVSN, the only armed force that had not dissolved after the armistice and had maintained its alliance with the Germans; he lamented how the MVSN had been merged completely into the National Republican Guard together with the Carabinieri and the PAI, losing its military character to become what was "essentially a police force".

In January 1944 he was appointed judge in the Verona Trial against the members of the Grand Council of Fascism who had voted the order of the day which had resulted in the fall of the Mussolini government on 25 July 1943.

Montagna argued that Marshal of Italy Emilio De Bono should not be sentenced to death, but his efforts were frustrated by the opposition of the intransigent Enrico Vezzalini.