Henry Garnet Forrest was born in Brunswick, a suburb of Melbourne in December 1895. Before the first world war he, like Harry Cobby, worked as a clerk in Melbourne. His original enlistment as an infantryman was soon changed to the Australian Flying Corps (AFC), and he was one of the first Australian pilots to be sent to France during the Ypres campaign. There he took part in operations with 43 and 32 squadrons Royal Flying Corps (RFC).

In November 1917 he was posted to 2 squadron which was equipped with the DH.5. These machines designed by Geoffrey DeHaviland were solid but poor performers that due to the reverse stagger of their wings made the pilot unable to see to the rear, a deadly fault in a fighter. This caused the machine to be used mainly in the army co-operation role.

Despite it's design shortcomings, 2 squadron also used them as interceptor fighters, and on December 1st Forrest claimed a large twin, probably a Gotha bomber, damaged. During the same month, 2 squadron started replacing their DH.5s with the new SE5.a, and commenced re-training.

The SE5a was a quick scout type aircraft with a liquid cooled inline engine rated at 200hp. It made a superbly stable gun platform and was as easy and forgiving to fly as the Sopwith Camel was difficult. It had a slight speed advantage over the Camel, and was capable of absorbing a great deal of punishment. Such pilots as Mannock and McCudden made their names in them, and even Albert Ball who originally loathed them finally came to appreciate their qualities as he scored most of his kills in the type.

As a high altitude fighter the SE5a took some getting used to after the DH5, but the pilots of 2 squadron soon adapted and in June 1918 joined the 10th Army wing and 4 squadron AFC at Bruay. Operations continued successfully though Forrest was unable to score until March 22nd.

On that day, the day following the opening of the German offensive that centred upon St. Quentin, eight SE5a scouts of 2 squadron attacked a group of two seaters that were being protected by Albatross DV scouts. Lieutenant Robert McKenzie, a former chemist and medical corpsman, sent one down, whilst Forrest flamed a two seater. A short time later on the same patrol, more Albatross DV fighters were encountered, and Forrest managed to shoot two of them out of the sky, with Captain Leslie Holden (who after the war went on to find the missing "Southern Cross" and Kingsford Smith in 1929), accounting for a third. This made a score of six for no losses on one patrol!

The following day Forrest scored another two seater as the campaign continued. At the end of March 2 squadron joined 43 and 80 squadrons RAF to form the Bellevue circus. A circus was the term given to the elite units that travelled up and down the front to give air power and superiority to where the ground troops needed it most. It was originally a German idea, but they used their best pilots to form new units. These pilots were lionised in the press and encouraged to paint their machines in fanciful colours and patterns, hence the originally derogatory term "circus". The most famous German circus was headed by the Red Baron, and soon the British took up the concept, using whole units instead of individuals. Being British they decorated their machines soberly, causing 2 squadron to cease using their widely recognised boomerang insignia, and carry a white stripe instead.

On April 2nd, Forrest shared a DKW two seater with Pilot Officer Roby Manual (his opening score). Circus operations continued throughout the remains of April and into May, but the German pilots wisely avoided such large concentrations of British machines, and combats were uncommon. Despite this, Forrest managed to bag another two seater on May 8th.

From mid May the Germans started appearing in formations of up to fifty aircraft, and despite being outnumbered the British formations always rose to the challenge, with 2 squadron scoring very well on June 1st and 2nd by downing twelve aircraft, Forrest accounting for three of them.

Another move on July 1st brought 2 and 4 squadrons AFC to Litters where they joined forces with 46 and 103 squadrons RAF to form 80 wing. Forrest made his mark immediately by downing two Pfalz DIIIs and a Fokker DrI triplane on the 2nd. This was however the last aerial combat Forrest was involved in, and he was posted to England on July 19.

The SE5.a continued in service with the newly formed RAAF into the early twenties, and upon retirement of the machines one was painted to represent Forrest's machine, and presented to the Australian War Memorial in Canberra. It is currently on display in the aircraft hall.