But how does an actual invasion of Belgium go over with the civilians, and not the warmongers?
It's going to be a hard sell to claim that to protect the Independence of Belgium, that the King and Government of Belgium must be overthrown.
Perfidious Albion, indeed
Well, that rather depends upon whose telling the story. Sell it as a "they are acting under duress" along with "The Kaiser's aim is to see this flag (photo of Imperial German standard) flying all over the World*" and "The High Sea Fleet based on Belgian ports" scare. Or - as I believe has happened occasionally - just outright lie!
The British didn't build a global empire without occasionally being perfidious.
*This was a quote from the contemporary "The Great War" by HW Wilson. My grandmother had the original 13 bound volumes and this quote from the first volume has remained with me for over 50 years. It was interesting to read the original British version of Jutland, then in the last volume an analysis of the actual results, along with an admission that the loss of the dreadnought
HMS Audacious in 1914 was known of but covered up by the British press, even though plenty of US tourists on the
SS Olympic had taken photos of the foundering battleship with their box brownies.