Leaflet
Explanation
The Joke
The comic shows a person in military-style gear (helmet, green clothing) crouched in a trench or behind a barrier, holding a paintball gun. In the second panel, an unfolded leaflet is visible that reads: "WHILE YOU FIGHT, YOUR SWEETHEART SMOOCHES OTHERS!"
The caption at the bottom reads: "My favorite paintball tactic is demoralizing leaflet propaganda."
The Humor
The humor comes from applying a real wartime psychological warfare tactic -- dropping propaganda leaflets to demoralize enemy troops -- to the absurdly low-stakes context of a paintball game. In actual wars, militaries have historically dropped leaflets over enemy positions telling soldiers that their wives and girlfriends are being unfaithful back home, hoping to sap their will to fight. Transposing this grim and manipulative tactic into a recreational weekend paintball match is inherently funny because of the massive gap between the seriousness of the method and the triviality of the situation.
The specific message on the leaflet ("your sweetheart smooches others") is written in an almost quaintly old-fashioned way, reminiscent of actual WWII-era propaganda language, which adds another layer of comedy.
References
Leaflet propaganda has been used in virtually every major conflict since World War I. During World War II, both the Allies and Axis powers dropped billions of leaflets. A common theme was targeting soldiers' anxieties about infidelity back home. The comic directly parodies this specific genre of psychological operations (PSYOP) material.