。
NATO bombed the Bosnian Serb bases and other targets at that time.
Literal
NATO [topic-は] that-time [at-に] Bosnia [genitive-の] Serbs [genitive-の] bases etc [object-を] aerially-bombed.
News/history report. 空爆する ('to aerially bomb,' 'to conduct an airstrike') is a Sino-Japanese compound: 空 (sky/air) + 爆 (bomb). など ('etc.,' 'among others') hedges the list of targets — the bombing included bases and other unspecified targets. The そのとき ('at that time,' 'then') situates the event in a previously established temporal frame. 'Bosnian Serbs' is rendered as ボスニアのセルビア人 — literally 'Bosnia's Serb people.' This could describe NATO's intervention in the Bosnian War in the mid-1990s (Operation Deliberate Force, 1995).