How can I view email performance segmented by country?

In the Setup section of an Email Performance report, you can add "Group by Segmentations." You would have to first create a segmentation for the countries you want to see--which unfortunately is a manual process.

I don't think you can do it any other way, since it doesn't let you group by a field like you would a Lead Performance report.

You can also constrain using a Smart List where Country = X, which will only show you activity for that country+email, assuming that email went to that country.

Thank you Matt. When I select group by segmentation, it's says 'select' in the field. When I click on it, its blank. When I type in the segmentation name, and hit apply, it closes the box but the segmentation isn't applied to the list. 

Did you make sure to create and approve your segmentation in the Lead Database section first? Here's instructions: https://community.marketo.com/MarketoTutorial?id=kA250000000KyzyCAC