What army is invading Pepperland in the Beatles film "Yellow Submarine"?

"Yellow Submarine" (also known as "The Beatles: Yellow Submarine") is a 1968 British animated musical fantasy comedy film inspired by the music of the Beatles, directed by animation producer George Dunning, and produced by United Artists and King Features Syndicate. Initial press reports stated that the Beatles themselves would provide their own character voices; however, aside from composing and performing the songs, the real Beatles participated only in the closing scene of the film, while their cartoon counterparts were voiced by other actors.

Pepperland is a cheerful, music-loving paradise under the sea, protected by Sgt. Pepper's Lonely Hearts Club Band. The titular Yellow Submarine rests on an Aztec-like pyramid on a hill. At the edge of the land is a range of high blue mountains.

The land falls under a surprise attack from the music-hating Blue Meanies, who live beyond the blue mountains. The attack starts with magical projectiles fired from big artillery stationed in the blue mountains. The Blue Meanies seal the band inside a music-proof blue glass globe, they render the Pepperlanders immobile as statues by shooting arrows or dropping giant green apples upon them (a reference to the Apple Records music label), and drain the entire countryside of color.

The Beatles were not enthusiastic about participating in a motion picture, They saw an animated film as a favorable way to complete their commitment to United Artists, however.