the subject of great public interest
I can not understand the last sentence of the following context: " believing the subject of great public interest". I can not understand "who believe(s)" and "what they believe".
Before going overseas, Henry Carr painted scenes in the capital throughout the blackout, including such public spaces as St Clement Danes, St Pancras Station, and the Underground. He was desperate to see that war artists recorded the everyday toil of servicemen and women, believing the subject of great public interest.