Immediate capturing of capital

This person took my capital as an ally, but when I ended alliance and declared war on him it showed that he captured my capital and I had lost. Is this a bug?

It's an accepted game mechanic for now... once some of the bigger issues are resolved, this would be good to revisit.

Sadly brianwl is right.

There are two options to avoid this situation from happening again:
1.Don't ally anyone.

2.Ally but have one unit in each one of your cities.

Bonus tip they can take your cities with any unit if they are empty no matter if it will be a bomber or an infatry.