Guadalcanal was the first land battle in the Pacific War between USA and Japan. The film was released the same year as the events. Sure, there is plenty of patriotism and propaganda, but for a Hollywood combat film made in the war years, this is relatively understated, and fairly realistic. It warns that the Japanese would be a formidable enemy and any victories for the US military would be hard won.
This isn't war as hell. The film is intended to boost morale and reassure the homefront. Guadalcanal Diary is narrated by the character of a war correspondent, and it was based on a book by a Richard Tregaskis, a journalist who was reporting back for a news wire service. It follows the US Marines for six months, from landing on the beaches to being relieved by the US army. The voice over contributes some strategical and philosophical context.
The story mainly focuses on the ordinary soldiers, who are presented as classless and multi-ethnic. It's a vehicle for a range of fine ensemble performances. William Bendix stands out as a brave, determined but slightly dim GI Joe from Brooklyn, a role he would play a few times. These men are not really fighting for strategic gain or country, but for each other. And to survive.
The film has a practical message for the homefront: send mail, give blood and buy war bonds. It warns them of what their returning soldiers will have experienced. By rowing back on the heroics, the film feels even more moving because it is credible. It shows death and suffering and told their families, and people of the future, that their sacrifice was worthy of respect.