WARSAW, Poland — A number of Eastern European countries are gearing up to boost their Soviet-designed air defense systems, though their motivations for the investments vary.

While Russia’s activity in Ukraine represents a major factor accelerating the procurements of certain nations — as shown by Poland and Romania’s Patriot acquisitions — regional rivalries constitute a second driving force, as exemplified by Serbia and its tense relationship with Croatia.

In Romania, which has positioned itself as one of the leading Eastern European allies in terms of defense procurement over recent years, a large share of military expenditure focuses on the acquisition of Raytheon’s Patriot air and missile defense system. In November 2017, Bucharest inked a letter of offer and acceptance to pave the way for the system’s purchase under a contract valued at up to $3.9 billion. To demonstrate the system’s capabilities over the Black Sea, the United States and Romanian military held a joint exercise in June.

George Scutaru, a lawmaker for the Romanian National Liberal Party and the director for development at the New Strategy Center think tank in Bucharest, told Defense News that “Romania wants to show determination in the process of increasing its own defense capabilities” and contribute “to NATO’s effort to discourage Russian actions in the Black Sea region.”

Scutaru also noted that Romania signed a contract with Lockheed Martin to buy both the High Mobility Artillery Rocket System and the Guided Multiple Launch Rocket System for $1.25 billion, making it the first European HIMARS customer.

“Romania’s posture and responsibility remains crucial between the Black Sea, western Balkans and the eastern flank cohesion,” he said.

Locked in an arms race with Croatia, Serbia has positioned itself as a neutral state capable of purchasing weapons and military equipment both from NATO and the Russia-led Collective Security Treaty Organization military alliance.

Serbia acquired Mikoyan MiG-29 fighters from Belarus and Russia, and it expressed interest in buying Russian-made S-300 air defense systems. However, the Serbian government also ordered nine H145M helicopters from Airbus, with the first delivered last June.

One month later, the Serbian government in Belgrade signed a deal to acquire 18 Mistral 3 short-range air defense systems fitted with 50 missiles from European missile-maker MBDA.

“With this first European missile order, Serbia becomes the 32nd customer country for the Mistral missile and the 10th country invited to join the Mistral user club,” MBDA said in a 16 July statement. “As the latest generation of Mistral family today in service, Mistral 3 features a very high resistance to infrared countermeasures and a capability to engage air targets presenting a low thermal signature, such as missiles and UAVs.”

The deal comes as Serbia’s regional rival, Croatia, is developing a program to upgrade its Air Force and acquire new fighter jets. Earlier this year, Croatia’s cabinet canceled its previous decision to purchase used F-16C/D Barak fighters from Israel amid reports of U.S. government opposition to the contract.

In addition to Israel and the United States, other bidders included Greece, which offered its used F-16 aircraft, and Sweden, with its JAS 39 Gripen fighter jets.

“The political and military establishment in Belgrade claims that the acquisition of new weapons is a function of the balance in the region, and that it is a deterrent,” analyst Duro Kozar wrote in the Sarajevo-based Oslobodenje daily. “This said, in this neighborhood in which most countries are NATO member states, this does not bother Serbia at all to ‘sharpen its teeth’ and have a much larger arsenal than a military neutral state would need.”