Sort into groups without any hints

1.) There are 5, groups with 6, 5, 4, 3 and 2 countries each. 2.) No hints are given except for the number of countries in each group. 3.) Guess the common attribute based on the number of countries in the group. 4.) Sort each country into its correct group 5.) Countries falling under one group will not fall under any other group 6.) Conversely, all countries fall under some group