What nearly tipped me off my chair was the pop up asking me if I wanted to declare war on the Barbarians! I'm currently operating under a Republic, and the Barbs have recently taken over the Spanish city of Barcelona, but even then there seems to be no good reason for this message to pop up. Never seen it before.

I had two options: Cancel action or declare war. After some hesitation, I chose the latter. The Hawk Party derailed Senate interference and I was able to collect the 150 gold.

In my game, I remember that it occured straight after I'd established an embassy with a Barbarian city. In the embassy screen, it said they'd made 'Contact' with me. Turn after I sent some units to take the city, and got the option to declare war.

Another occasion, as I recall, was when there had been barb units in my territory for a few years. There was no leader, they weren't bothering me, so I basically ignored them. Ultimately, when I did move against them, I received the message. Now, they hadn't taken one of my cities. It's possible, of course, that they'd taken another civ's city elsewhere on the map. On the other hand, there have been many occasions when I know they've taken other civs' cities, and I haven't received the message as I've moved to attack.

All of which means, I guess, like so many other things in this great game, that sometimes there's a pattern, sometimes there isn't.

1) The Barbarians must, for all we know, have established themselves with at least one city

and

2) The human player will have to make the "first strike" against the Barbs, i.e. the Barbs haven't previously attacked your civilization, hence there hasn't technically been a state of war between you and the Barbs. Since you're the "agressor" operating under a representative government, you get the option to cancel the action - as you would if making a first strike against an AI civilization.

So, Barb city, first strike, representative government... three prerequisites.