The Russians are always big on big parades of military hardware. And the 70th anniversary of Victory in Europe Day is a fine excuse for a parade. And of course, the military being the military, they have to hold a rehearsal.
I thought adding in the T-34 tanks was an especially nice touch.
By way of contrast, you’ll notice the US pretty much never does anything like this. Can you imagine the 1st Armored Division rolling through DC just to show off?