Jiāngjūn yīshēnglìngxià, shìbīng men fēnfēn shàngmǎ chūzhēng.
At the general's command, the soldiers all mounted their horses and set off for battle.
Tips
usage
上马 has a vivid literal image (mounting a horse) and a very common figurative extension: launching or starting a major project or initiative. The contrast 下马 (xiàmǎ) means to halt or cancel a project.