1- The only way to go from Turkey to Ukraine by crossing only 2 European countries is to go through Bulgaria and Romania :

B1 = Bulgaria, C1 = Romania

2- The only other European country adjacent to Turkey is Greece A2 = Greece

3- The only country adjacent to Greece and Bulgaria is Macedonia B2 = Macedonia

4- The only remaining country that borders Greece and that is not already in a box is Albania A3 = Albania

5- The only country adjacent to Macedonia and Albania is Kosovo B3 = Kosovo

6- The only countries bordering Macedonia and Romania are Serbia and Bulgaria (already in B1) C2 = Serbia

7- The only countries bordering Serbia and Kosovo are Montenegro and Macedonia (in B2) C3 = Montenegro

8- The only countries bordering Serbia and Ukraine are Hungary and Romania (in C1) D2 = Hungary

9- The only countries bordering Hungary and Montenegro are Croatia and Serbia (in C3) D3 = Croatia

10- The only remaining countries bordering Croatia are Bosnia and Herzegovina and Slovenia. Bosnia and Herzegovina is not possible for E3 because all its neighbours are already present in the puzzle (Montenegro, Serbia, Croatia), and E2 must be bordering E3.

E3 = Slovenia

11- The only countries bordering Slovenia and Hungary are Austria and Croatia (in D3) E2 = Austria

12- The only countries bordering Austria and Ukraine are Slovakia and Hungary (in D2) E1 = Slovakia