Complex sentence: Hidenobu followed in serving under Ishida Mitsunari during the Sekigahara Campaign of 1600, in which he controlled the Mino Castle, an important element in Ishida's overall plans, but ended up losing it to an attack by Ikeda Terumasa and Fukushima Masanori.
Split into simpler sentences: Hidenobu followed in serving under Ishida Mitsunari during the Battle of Sekigahara in 1600, during which he controlled the Mino Castle, an important element in Mitsunari's overall plans. However, he ended up losing during an attack by Ikeda Terumasa and Fukushima Masanori.