Croatia, Bosnia and Herzegovina, Montenegro, Albania, Serbia, Kosovo, Macedonia, Greece, Turkey, Bulgaria, and the Ukraine all have land that is located on the Balkan Peninsula; a total of 11 countries.
Andorra, Portugal and Spain are on the Iberian Peninsula. Denmark is on the Jutland Peninsula; Italy, San Marino and Vatican City are on the Italian Peninsula. Albania, Bosnia, Bulgaria, Greece, Kosovo, Macedonia and Montenegro are on the Balkan Peninsula. Turkey is on the Anatolian Peninsula. North and South Korea share the Korean Peninsula.
Norway and Sweden share the Scandinavian peninsula.Norway, Sweden.
Sweden, Norway, and Finland all are on the Scandinavian Peninsula.
Portugal and Andorra are the two countries on the Iberian Peninsula that share a border with Spain.
There are many countries that are a peninsula: -Italy -Denmark -Greece -Turkey -South Africa -India -South Korea *the listed countries are only countries who are they, in themselves, a peninsula. There are much more countries that are part of a peninsula but not wholly one (i.e. Spain)*
The biggest similarity between Albanians and Greeks, Macedonians and Serbs are the Balkan genetics that are common to the area.
Norway and Sweden share the Scandinavian peninsula
Spain and Portugal. If you consider the Iberian peninsula to begin at the Pyrenees mountains (los Pirineos) then you should include the Principality of Andorra in your list.
The Iberian Peninsula
Spain Peninsula Iberica
Spain and Portugal share the Iberian Peninsula.