Those flags represent the principal participants on the side of the Entente during WWI (and yes, Japan climbed in on ‘our’ side within a month of the war commencing… largely in the hope of making territorial gains).
At the opening of WWI the balance of power in Europe was divided between the “Triple Entente” (Britain, France, and Russia) and the “Triple Alliance” (Germany, Austria-Hungary, and Italy).
When the conflict started, the Entente powers were joined by Italy (who switched allegiance), Romania, Japan and finally the United States plus associated members including (at various times): Armenia, Belgium, Brazil, China, Costa Rica, Cuba, Greece, Guatemala, Haiti, Honduras, Montenegro, Nicaragua, Portugal, Panama, Serbia and Siam among others.