An unusual military build-up in southern Europe sparked fears of another invasion like Russia's full-scale assault on Ukraine

• Tensions have spiked between Serbia and neighboring Kosovo after a deadly shooting in September.
• The US warned last week that Serbia staged an "unprecedented" military build-up along its border.
• Kosovo's top diplomat compared the situation to what Russia did before its invasion of Ukraine.

A massive build-up of military power in Serbia has officials in neighboring Kosovo drawing comparisons to what Russian forces were doing before Moscow's full-scale invasion of Ukraine. 

The Biden administration said last week that it has observed a "worrying cycle" of rising tension and sporadic violence over the past few months between the two southern European countries, which have long been at odds and maintain a frosty relationship that's rooted in ethnic divisions. Tensions skyrocketed in late September after heavily armed Serbian gunmen staged a deadly attack in northern Kosovo, and in the aftermath, US officials said they were observing an "unprecedented" mobilization of Serbian firepower along its border with Kosovo. 

"There has never been this kind of concentration of troops in recent years," Kosovo's Foreign Minister Donika Gervalla-Schwarz told German broadcaster Deutschlandfunk on Monday, according to Reuters. "The weaponry they have there, the tanks — it gives us a bad feeling because we don't know how the international community will respond."

Gervalla-Schwarz added that Serbia's rhetoric and "methods" are similar to what Russia did to Ukraine — specifically the build-up of forces along their shared border in the months leading up to Moscow's eventual full-scale invasion of its neighbor in February 2022. Russian President Vladimir Putin, an friend of leadership in Belgrade, repeatedly downplayed the situation and military escalation at the time. Russia repeatedly denied it was planning to invade. Serbia, likewise, has tried to pour cold water on the fears of an invasion.

The majority of Kosovo's population consists of ethnic Albanians. Many of the country's ethnic Serbs — a minority — live in the north. After Yugoslavia broke apart in the 1990s, Kosovo moved to seek independence from Serbia, which responded by cracking down on the Albanian population during a bloody war that lasted from 1998 to 1999, when NATO eventually intervened with a bombing campaign that pushed Belgrade's forces out. In 2008, Kosovo unilaterally declared independence from Serbia, and while the latter has refused to recognize this move, nearly 100 other countries — including the US — have done so. 

Although active fighting stopped more than two decades ago, tensions between the countries still persist in northern Kosovo, despite efforts from the West to normalize ties. 

On September 24, heavily armed Serb gunmen killed a Kosovar police officer and stormed a monestary in northern Kosovo, setting off a shootout that left several attackers dead. The incident, which marked one of the worst escalations in years and was condemned by Western countries, raised fears that the situation between Belgrade and Pristina could seriously deteriorate and trigger more violence.

In the wake of the increasing tensions, NATO announced plans to increase the size of its Kosovo-based peacekeeping mission — known as Kosovo Force, or KFOR — with several hundred additional soldiers from the UK. KFOR was established in 1999 after NATO ended its months-long bombing campaign of Serbia and is designed to deter hostility against Kosovo and maintain security in the region, according to its mandate. 

"We call on all parties to urgently de-escalate," NATO Secretary General Jens Stoltenberg said in a Friday statement. "We continue to urge Belgrade and Pristina to engage in the EU-facilitated dialogue, as the only way to resolve outstanding issues and reach solutions that respect the rights of all communities. This is key for lasting security in Kosovo and stability in the region."

Meanwhile, on Friday, the US made public its observation of the Serbian military build-up along the border and expressed concerns over the situation. 

"We are monitoring a large Serbian military deployment along the border with Kosovo that includes an unprecedented staging of advanced Serbian artillery, tanks, and mechanized infantry units. We believe that this is a very destabilizing development," White House National Security Council Spokesperson John Kirby told reporters on Friday, calling on Belgrade to withdraw its forces. 

Serbia later announced on Monday that it had pulled back around half of its troops from the border. Kosovar officials have said their country remains on high alert.  

Albin Kurti, Kosovo's prime minister, wrote in a post on X, the social media platform formally known as Twitter, that local authorities had obtained documentation and "confirmed" that the deadly September attack "was part of a larger plan to annex the north of Kosova via a coordinated attack on 37 distinct positions. Establishing a corridor to Serbia would follow, to enable supply of arms & troops." Serbia's leadership has denied any involvement in this incident.