I did something similar earlier this week with a few restaurants in the Redlands area (instead of countries, for example). It was fairly easy to set up as long as you have a complete data set to work with.
First, you need to create an ArcGIS Online Web Map with the data you desire (http://jsapi.maps.arcgis.com/home/index.html). My data was in CSV format but there are better options for larger data sets.
After that, you can simply replace the webmap-id used by the sample with your own map's webmap-ID.
It should show all of the information from your dataset regarding a specific country in the sidebar.
As for custom content in the left pane, you could create a template for the side panel to change what information is displayed based on a specific property of a particular country within your dataset.
If I missed anything, I apologize. Good luck and I hope this helps.
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