Masiero began aviation schooling on 1 February 1915. On 1 August 1915, he was rated a qualified pilot on the
Maurice Farman aircraft. He completed pilot's training and was assigned as a
sergente to
7a Squadriglia (later to be redesignated
26a Squadriglia) reaching the front on 15 October 1915. He qualified as a
Voisin pilot on 15 November 1915. In December 1915 Masiero began to fly a Voisin for
26a Squadriglia. He flew 58 reconnaissance sorties over the
Carso and sometimes returned with battle damage. At some point prior to February 1917, Guido Masiero was
commissioned as a
sottotenente. After the squadron was disbanded on 4 March 1917, Masiero was hastily reassigned to
103a Squadriglia at
Brindisi. He then transferred to the
Nieuport training unit at
Malpensa on 14 May 1917. On 29 July 1917, he was promoted to
tenente. Next, on 10 September 1917, he was forwarded to the
86a Squadiglia at
Ponte San Pietro for further training on the new
Ansaldo SVAs. As the SVAs were still under development, Masiero was sent to
91a Squadiglia on 27 October 1917. This was cut short by demands for all pilots to fight in the
Battle of Caporetto; Masiero arrived on 2 November 1917 with
78a Squadriglia. On 7 November 1917, Masiero staked his first victory claim, but it went unconfirmed. He then joined with
Mario Fucini, Razzi, and
Antonio Chiri for his first official victory on 13 November 1917. Six days later, he scored a solo win, but his second claim on the 19 November went unconfirmed, as did one submitted for the following day. It was not until 10 December 1917 that he scored again, to balance out his count of confirmed and unconfirmed wins at three each. Four days later, another claim went unconfirmed. Finally, on 26 December 1917, in two separate fights, Masiero became an ace, sharing both victories with a number of other Allied pilots. Masiero was promoted to
tenente from
sottotentente on 17 January 1918. He would go on to make six more victory claims during the first six months of 1918, to no avail. On 29 March 1918, he left
78a Squadriglia for posting to
3a Sezione SVA to fly the new Ansaldo SVAs. A review committee meeting immediately postwar disallowed at least six previously confirmed victories, leaving Masiero's tally at five confirmed. ==Post-World War I==