Kosovo has closed its largest border crossing after protesters blocked it amid mounting tensions between Serbia and Kosovo. Serbia said Tuesday its army has been put on high alert following weeks of protests and roadblocks. What started as a dispute over car license plates is now threatening to boil over, with Kosovo accusing Serbia of attempting to destabilize the country under the influence of Russia. Some 50,000 ethnic Serbs live in northern Kosovo but do not recognize Kosovo’s independence. The arrest of a former Serbian police officer in northern Kosovo earlier this month has refueled the simmering tensions.