Another masterpiece, which I have had the privilege to examine closely in the Rijkmuseum, Amsterdam. A brief historic review is available on Wikipedia and Britannica also offers a great and free article on the artist and this painting. A must read.
Any review and interpretation would inevitably be trimmed, because so is the piece in its current condition. Still, we can discuss what is at hand as if it were a complete work of art. The most salient compositional feature of this group portrait is the clustering: in what appears to be Rembrandt’s trademark device, the crowd is divided into several groups, each with its own distinct dynamic. Additionally, the groups relate to each other to create a complex and truly grand composition. Each smaller congregation may be viewed as a miniature separate painting that conceptually echoes the overall design, a local person representing a group and the local central figure the lavishly illumined captain, around whom the groups gather. Such symmetry makes particular sense in the militant context of the scene, reflecting the notion of the chain of command.
Three groups encompass the two central figures and we can presume that a lieutenant dominates each of these subordinate gatherings. Weapons and accessories connect between them: a pike, a rifle (probably a musket) and a flag — an original compositional adhesion that also serves as a symbolic reminder of the nature and possible consequences of tasks assigned to the town militia. These men are brothers in arms, literally linked by their weapons, and just as they point them towards each other here, in a simulative display, they would do so to confront a real enemy on an actual mission. One of the most striking features of this piece is the general mood of apprehension and awareness of a mutual goal, transpiring despite the appearance of everybody being focused on themselves, either their thoughts or equipment. Composition overcomes private distractions, and brings everything together, making the painting an exemplar of unity.
I cannot find any explanation as to why the girl on the left is alighted as strongly as the captain. Perhaps she was intended to be a part of the narrative which is now missing. One could imagine that she is the daughter of one of the portrayed, or the messenger who caused the whole bustle. In the first case, the light discloses a kinship with the similarly illumined commander, whereas in the second it emphasizes the importance of the messenger — of knowledge and information — as equal to that of dealing with it later on. Finally, I would also mention the strong illusion of sound: cleaning of armament, talking and arguing, clinking of metal and last but not least the beating of the drum. Noise completes the scene, imbuing it with a somewhat cinematic element and adding another touch of realism, which still, as a group portrait (and cinema) requires, is relied on acting and posing.