The Kosovo War has long since been forgotten by the American public.
After all, it was supposed to be over. The good guys won, the bad guys lost, and everything got settled.
The problem is that all three of those assumptions are wrong.

Kosovo’s Parliament overwhelmingly approved legislation on Friday to form an army, prompting criticism from NATO and European Union officials and angering neighboring Serbia, which said it was prepared to use its own army to protect ethnic Serbs in Kosovo.
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Aleksandar Vucic, Serbia’s president, said in the town of Trstenik in central Serbia on the eve of the vote, “Not a single act in the international law gives them the right to form an army.”

“Everything that Pristina does — and evidently it does it all with support of the U.S. and Britain — is against the law,” Mr. Vucic added.
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Serbia’s prime minister, Ana Brnabic, said the formation of a Kosovo army ran counter to efforts at stability in the volatile Balkans, according to The Associated Press. She added that she hoped Belgrade would not have to use any of its 28,000 troops to protect the Serbian minority in Kosovo, although “this is currently one of the options on the table.”

Nearly 20 years after the war, about 5,000 NATO troops are in Kosovo, including some 600 American soldiers. According to Kosovo’s Constitution, NATO is the only armed force allowed to operate there.
Interestingly, NATO and the U.S. are not on the same page with this development.

Jens Stoltenberg, NATO’s secretary general, had warned the government in Pristina, the Kosovo capital, that the country would “face serious consequences.”
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But the United States backed the formation of an army. The American ambassador to Kosovo, Philip Kosnett, previously said that it was “only natural for Kosovo as a sovereign, independent country to have a self-defense capability.”
Mr. Kosnett said in a Twitter message on Friday, “The U.S. will be there with you.”

The reasons why Serbia is so hostile to an armed Kosovo are numerous and go all the way back to just before NATO attacked Serbia.
It all started with a lie.

Amid indications that "genocide is unfolding in Kosovo," the U.S. State Department on Monday welcomed a new Russian diplomatic effort to resolve the crisis, but said the only way to stop NATO airstrikes against Yugoslavia would be for President Slobodan Milosevic to accept a U.S.-brokered peace plan.

Two years later a UN-supervised Supreme Court in the Kosovan capital, Pristina ruled "the exactions committed by Milosevic's regime cannot be qualified as criminal acts of genocide, since their purpose was not the destruction of the Albanian ethnic group..."
That ruling was mostly ignored, but that wasn't the big, enduring fact that continues to haunt Kosovo today. That fact was that our allies were/are terrorists.

At the time, British Prime Minister Tony Blair openly described the intervention in Kosovo as “a battle between good and evil; between civilisation and barbarity; between democracy and dictatorship.” But the story was hardly so pure. The case for humanitarian intervention under international law was based on preventing more Serb atrocities, but in practice that meant supporting the Kosovo Liberation Army (KLA)—a group that U.S. officials had previously described as terrorist. It was fighting for full independence rather than Washington’s more limited goal of political autonomy. U.S. officials were aware that moralistic rhetoric cloaked political risks: Intelligence agencies privately warned that the KLA was trying to provoke Serbian massacres in hopes of persuading NATO to support its bid for independence.

Empowering terrorists has drawbacks.
For starters, Kosovo was the Number One nation for ISIS recruits in Europe. Those jihadists that were arrested in Kosovo are already being released, so you can see why Serbia might be a bit paranoid of an armed Kosovo.