berlin serbian u.s. paratroopers jump side-by-side joint exercise aim strengthen military tie serbia u.s. general charge nato ally air command say move could trigger protest moscow exercise take place invitation serbian government paratroopers countries jump side-by-side c-130j transport plan build lockheed martin corp so-called insertion exercise 100 u.s. air force u.s. army personnel participate event general tod wolters also oversee u.s. air force europe africa tell reuters actually get paratroop wing result activities confidence-building activities relationships last lifetime certainly enhance technical expertise serbs say immediately clear many serbian force would participate wolters say tensions balkans remain challenge nato u.s. military engagement key continue challenge get right command focus get right resources get right dialogue time tell unfold say nato-related activities serbia red flag russia worry nato expansion former communist east moscow also seek bolster military tie belgrade donation six mig-29 fighter jet serbia perform delicate balance act russia west reject call u.s. officials pick side large state emerge collapse yugoslavia 1990s orthodox christian slavic serbia natural affinity moscow keen join european union although eu serbia single large trade partner investor russia control oil gas supply serbian u.s. paratroopers earn 'wings bilateral military exercise