Complex sentence: On 12 June 1999, Blunt's troop led the 30,000-strong NATO peacekeeping force from the Macedonia border to Pristina International Airport, however a Russian military contingent had moved in and taken control of the airport before his unit's arrival.
Split into simpler sentences: On 12 June 1999, the troop led the 30,000-strong NATO peacekeeping force from the Macedonia border to Pristina International Airport. However a Russian military contingent had moved in and taken control of the airport before his unit's arrival.