Complex sentence: On 11 May, a 3,000 to 4,000 man strong FNLC rebel group arrived, accompanied by the 2nd Cuban Division; departing from Angola, it had crossed neutral Zambia.
Split into simpler sentences: On 11 May, a 3,000 to 4,000 man strong FNLC rebel group arrived. Departing from Angola, it had crossed neutral Zambia.