Playing order and military ranks
Its true that 先鋒 (Senpô), 次鋒 (Jihô), 中堅 (Chûken), 副将 (Fukushô) and 大将 (Taishô) could mean vanguard, sergeant (couldn´t find Jihô as a military rank), lieutenant, vice-captain, and captain respectively. But some of them could even mean other military ranks.
Each one of those terms means the order of the players from first to fifth (last position) in a team match (5 on 5 match competition). Examples are: A 5 on 5 match of jûdô, kendô, shôgi, competitive karuta (Uta-garuta´s game), or igo, known here as go. Or other games. Should be correct to say the Senpô is the first player nor the vanguard. The same goes to the other positions.
They´re players, not soldiers.