Judging from other description in the letter that he would keep staying at Unagi Hot Spring so as not to make his young Lord impetuous, the words 'I will decide to raise the army' might have meant defense and campaign against Russia, which was his biggest concern in those days, instead of starting a civil war.
ただ、書簡中では若殿輩（わかとのばら）が逸（はや）らないようにこの鰻温泉を動かないとも記しているので、この「立つと決する」は内乱よりは当時西郷が最も心配していた対ロシアのための防御・外征を意味していた可能性が高い。