Complex sentence: Kgosi Galeshewe, who was across the border in the Transvaal at the time trying unsuccessfully to get Boer support for any British attack was personally held responsible for "rebellion" and "murder".
Split into simpler sentences: Kgosi Galeshewe, who was across the border in the Transvaal at the time, was trying unsuccessfully to get Boer support for any British attack. He was personally held responsible for "rebellion" and "murder".