Moorcock describes this as a "funny, futuristic" book, but it is really a parody of the invasion scare sub-genre. I have read a few other books in that genre - The Riddle of the Sands by Erskine Childers, When William Came by Saki, and a collection edited by I.F. Clarke.
Moorcock is, however, correct to describe it as funny, despite the incomprehensible contemporary cultural references and unpleasant racial stereotypes (which as far as I remember are largely absent from later Wodehouse). England is invaded by nine different armies, ranging from the Germans and Russians down to the forces of Monaco and the Mad Mullah of Somaliland (the latter driven to further derangement by a meeting with Irish Nationalist leader John Dillon). The occupied English grumble about the disruption to cricket and the theatre caused by the invaders, but this is resolved when the German and Russian commanders agree to appear as music-hall acts.
Clarence Chugwater, the somewhat nerdy Boy Scout who is Wodehouse's comic hero, manages to sow dissension between the German and Russian leaders by way of his day-job at an entertainment weekly. The two armies come close to wiping each other out, the Boy Scouts capture the survivors, and England is saved. Hurrah! (In the unlikely event that anyone feels I have spoiled the ending for them, I would point out that all is revealed in the very first chapter.)
This is not a good starting place for reading Wodehouse's works. (Indeed, it wasn't even a very good starting place foir writing Wodehouse's works.) But it is an interesting intersection of the fringes of the sf genre with his rather different genius when both were at an early stage of development.