Almost all of Rivera's art told a story, many of which depicted Mexican society, the Mexican Revolution, or reflected his own personal social and political beliefs, and In the Arsenal is no different. It is based on a Corrido, a popular type of song in Mexico written by Rivera and called "So will be the proletarian revolution". We can see the words of the Corrido in the red band on the top.
The central figure is Frida Kahlo, who became Rivera's wife. Frida hands out weapons to revolutionary soldiers. To the far left Siqueiros. To the right Tina Modottiwith her lover Julio Antonio Mella, a Cuban revolutionary who fought the Cuban dictator Machado. A few years later Mello was shot dead while walking in the street arm-in-arm with Tina. Tina was accused of having hired the killer because she was said to have become tired of Mella. She was held by the police. But thanks to the influence of Rivera she was freed. The person with the black hat looking at Tina is Vittorio Vidali, a Stalinist agent, who became Tina's lover after Mella's death. There are strong indications that he was the actual killer of Mella.
The figures in this painting are an illustration of Rivera's transferring his political beliefs onto canvas. He was an active member of the Mexican Communist Party, and was friends with Leon Trotsky, who lived with him for seven months.