The painting depicts the events of the night of November 26, 1781, when British troops made a sudden attack (sortie) against the enemy batteries.
The death of the Spanish officer Don Jose de Barboza is the focal point of the painting.
He fell mortally wounded and died near his post refusing assistance after having been abandoned by his troops.
He is portrayed as rejecting the aid of General George Elliott, commander of the British troops.

In 1782, the siege was lifted, and Trumbull's friend Antonio de Poggi, an artist and dealer based in London who had been in the besieged garrison, told him of an earlier incident, which had occurred in November 1781.

This had all the ingredients he sought:
...the Heroism of the vanquished, the Humanity of the Victors - the darkness of night illuminating an extensive conflagration - the Hurry and Tumult of the troops busy in the work of destruction - the quiet & calm of the Officers, the guiding Spirits of the Scene.

Furthermore, Trumbull had been engaged in a series of paintings based on the American Revolution, which had been criticized in London.
He saw the subject of the Siege as one which he could demonstrate that he supported British heroism as well:
...and as I knew by painting them [his American history paintings], I had given offense to some extra-patriotic people in England, I now resolved to exert my utmost talent upon the Gibraltar, to show that noble and generous actions, by whomsoever performed, were the objects to whose celebration I meant to devote myself.

Trumbull labored on the composition, over many sketches and three large completed canvases.
As the project progressed, Trumbull's ambitions for it to be his big breakthrough to major patronage grew too.
He refused large offers for the picture, preferring to exhibit it privately for admission fees.

Horace Walpole, 4th Earl of Orford had called the painting: 
the finest picture [he] had ever seen painted on the northern side of the Alps.

The painting is depicted on the back of the 2010 Gibraltar 10-pound note.

The people highlighted in this composition are the dying José de Barboza and to his right and from left to right: Ensign A?.
Mackenzie (in Highland dress), Governor Eliott, Lt G.F. Koehler, Lt.
Col J. Hardy, Brig.
Gen C. Ross, Capt A. Witham, Capt Roger Curtis, Lieutent Thomas Trigge, Lt Colonel Hugo.
The painting depicts the events of the night of November 26, 1781, when British troops made a sudden attack (sortie) against the enemy batteries. The death of the Spanish officer Don Jose de Barboza is the focal point of the painting. He fell mortally wounded and died near his post refusing assistance after having been abandoned by his troops. He is portrayed as rejecting the aid of General George Elliott, commander of the British troops.
In 1782, the siege was lifted, and Trumbull's friend Antonio de Poggi, an artist and dealer based in London who had been in the besieged garrison, told him of an earlier incident, which had occurred in November 1781.
This had all the ingredients he sought:
...the Heroism of the vanquished, the Humanity of the Victors - the darkness of night illuminating an extensive conflagration - the Hurry and Tumult of the troops busy in the work of destruction - the quiet & calm of the Officers, the guiding Spirits of the Scene.
Furthermore, Trumbull had been engaged in a series of paintings based on the American Revolution, which had been criticized in London. He saw the subject of the Siege as one which he could demonstrate that he supported British heroism as well:
...and as I knew by painting them [his American history paintings], I had given offense to some extra-patriotic people in England, I now resolved to exert my utmost talent upon the Gibraltar, to show that noble and generous actions, by whomsoever performed, were the objects to whose celebration I meant to devote myself.
Trumbull labored on the composition, over many sketches and three large completed canvases. As the project progressed, Trumbull's ambitions for it to be his big breakthrough to major patronage grew too. He refused large offers for the picture, preferring to exhibit it privately for admission fees.
Horace Walpole, 4th Earl of Orford had called the painting: 
the finest picture [he] had ever seen painted on the northern side of the Alps.
The painting is depicted on the back of the 2010 Gibraltar 10-pound note.
The people highlighted in this composition are the dying José de Barboza and to his right and from left to right: Ensign A?. Mackenzie (in Highland dress), Governor Eliott, Lt G.F. Koehler, Lt.Col J. Hardy, Brig. Gen C. Ross, Capt A. Witham, Capt Roger Curtis, Lieutent Thomas Trigge, Lt Colonel Hugo.
The Sortie Made by the Garrison of Gibraltar