Franco-Prussian War.

[stormbringer3]

What do the green stars mean?
Thanks.

[BillRunacre]

The powerful fortress of Metz can be besieged by German forces and forced to surrender without the need to storm its defenses directly.
To do this, the Germans must capture all of the locations surrounding the fortress that have been marked by green stars, and ensure they have six units located within four hexes of hex 90,47. Once these conditions are met, there
will be a 20% chance each turn that all French units in and around Metz will surrender.
Historically the surrender of Metz on October 27, 1870 resulted in the capture of over 190,000 French soldiers, who almost certainly represented France’s last hope to achieve victory following the disastrous battle of Sedan.


The above is from the Strategy Guide, that can be accessed either via the Launcher or, while the playing the game, from one of the buttons at the top right of the screen.

It's worth a look as there is a lot of useful information in there!