Croatia is buying a US arms system. Serbia is buying Russian jets. Such moves could destabilise an already volatile region.
Not long ago, the Balkans were a hotbed of war and destruction.
Now, in a recent military upgrade, Serbia has received six new fighter jets from Russia.
Some analysts say it's a response to Croatia's plans to buy a new arms system from its NATO ally, the United States.
Purchases like these suggest an arms race could be developing in the Balkans.
The current rivalry between Serbia and Croatia dates to the collapse of Yugoslavia in the early 1990s.
Many Serbs are also hostile towards NATO after it bombed their country in 1999 to halt a crackdown on ethnic Albanians in Kosovo during the Kosovo war.
TRT World's Sally Ayhan went to Belgrade to get the Serbian perspective on developments in this volatile region.