Declaring war on a friend...

16 turns from a scientific victory as Germany, my declared friend Egypt just won a religious victory after converting enough of my cities. I'd seen this coming and tried to declare war to take out her religious units and buy some time, but as far as I can see this wasn't possible. Nothing in the diplomacy screen seemed to do it, nor did trying to move my military units on top of her religious/military units.

I'm at a loss here. It should be possible with my superior military to wipe her Apostles and Missionaries out and give me time to complete my Mars Colony components, but it seems mechanically impossible to declare war. We had been allies, but this had lapsed and not been renewed, and were just friends. The Civilopedia only says you can't declare war on allies.

Please help, I need my first Emperor/Scientific victories!