Senyu, also known as "Comrade in arms" or "War Comrade" was written supposedly right after the Russo-Japanese War in 1905. It has lyrics, so it probably wouldn't fit as battle music, but I'm posting it anyways for reference, and because it is a gripping song. Anywho, supposedly it was prohibited during the Second World War (most likely for the song's depressing and brutal, debatably anti-war depiction of war), but was apparently still very popular with veteran infantry, and tolerated by many officers.
Also nabbed some lyrics, they only encompass half the song though, and my are they saddening. I've also watched another video that seems to have slightly different lyrics, so I took elements of lyrics from both sources in an effort to create a smooth flowing transliteration. That being said, the lyrics won't be exact, and some may be out of place, but they will still illustrate the same meaning.
此處は御國を何百里 Koko wa o-kuni wo nanbyaku ri – Hundreds of leagues from our home country
離れて遠き滿洲の Hanarete tōki Manshū no – in far away Manchuria
赤い夕陽に照らされて Akai yūhi ni terasarete – Under the red glow of sunset
友は野末の石の下 Tomo wa nozue no ishi no ****a – My comrade rests in a far off corner of the field
思へば悲し昨日まで Omoeba kanashi kinō made – Sadly I recall the day before
真っ先驅けて突進し Massaki kakete tosshinshi – that he had bravely led the charge
敵を散々懲らしたる Teki wo sanzan kora****aru – our foe was severely beaten back
勇士は此處に眠れるか Yūshi wa koko ni nemureru ka – Will this brave warrior rest here?
嗚呼戰の最中に Aa tatakai no saichū ni – Ah in the very midst of battle
鄰に居ったこの友の Tonari ni otta kono tomo no – this comrade who was next to me
俄にはたと倒れしを Niwaka ni hatato taoreshi wo – suddenly was struck down, along with the flag he bore
我は思はず驅け寄って Ware wa omowazu kakeyotte – without a moment’s thought, I rushed to his side
軍律嚴しき中なれど Gunritsu kibishiki naka naredo – Military discipline is harsh
是が見捨てゝ置かれうか Kore ga misutete okaryō ka – but how could I have just left him lying there?
「確りせよ」と抱き起し "Shikkari se yo" to dakiokoshi – “Hang on!” I cried, raising him in my arms
假繃帶も彈丸の中 Karihōtai mo tama no naka – tending to his wounds with bullets still flying past us
折から起る吶喊に Ori kara okoru tokkan ni – Just at that moment our army’s charge broke through
友は漸う顏上げて Tomo wa yōyō kao agete - my comrade turned his face to the task at hand
「御國の爲だ構はずに "O-kuni no tame da kamawazu ni" – “It’s for the sake of the country, I’ll be fine.”
後れて呉れな」と目に涙 Okurete kure na" to me ni namida – “Do not tarry here” he said, tears in his eyes
後に心は殘れども Ato ni kokoro wa nokoredomo – Afterwards in spirit he may remain
殘しちやならぬ此の體 Nokoshicha naranu kono karada – But nothing would remain of him in body, I know
「それぢや行くよ」と別れたが "Sore ja yuku yo" to wakareta ga – “Then I shall go” I said and left him
永の別れとなつたのか Naga no wakare to natta no ka – but thus we were to part for eternity
戰濟んで日が暮れて Tatakai sunde hi ga kurete – After the fight and the sun went down
探しに戻る心では Sagashi ni modoru kokoro de wa – I returned to my comrade with a desperate heart
どうぞ生きて居て呉れよ Dōzo ikite ite kure yo – hoping to find him alive
物等言へと願ふたに Mononato ie to negōta ni - and able to speak
空しく冷えて魂は Munashiku hiete tamashii wa – But my wish was in vain, his body had lost its warmth and soul
故鄕へ歸つたポケットに Kuni e kaetta poketto ni – Already gone back to the Motherland
時計許りがコチコチと Tokei bakari ga kochikochi to – Only his pocket watch was still active
動いて居るも情無や Ugoite iru mo nasake naya – Tick-tocking to my intensified grief
思へば去年船出して Omoeba kyonen funade ****e – I met him last year on the ship
御國が見えず爲つた時 O-kuni ga miezu natta toki - On the sea of Genkai
玄界灘に手を握り Genkai-nada ni te wo nigiri - It was so far offshore the homeland was out of sight
名を名乘つたが始めにて Na wo nanotta ga hajime ni te – We shook hands and exchanged names
それより後は一本の Sore yori nochi wa ippon no – After, we got so friendly
煙草も二人分けて喫み Tabako mo futari wakete nomi – We shared cigarettes
着いた手紙も見せ合ふて Tsuita tegami mo miseōte – Even letter we received
身の上話繰り返し Minouebanashi kurikaeshi – And we shared stories of our respective lives
肩を抱いては口癖に Kata wo daite wa kuchiguse ni – We used to say to each other, hands around each-other’s shoulders
どうせ命はないものよ Dōse inochi wa naimono yo – “I have no hope to survive this war, so…”
死んだら骨を頼むぞと Shindara hone wo tanomu zo to - “Please take care of my bones.”
言い交はしたる二人仲 Iikawa****aru futari naka – So friendly we had been
思ひもよらず我一人 Omoi mo yorazu ware hitori – I had never imagined this would happen
不思議に命永らへて Fushigi ni inochi nagaraete – That only I would survive this war by fate
赤い夕陽の滿洲に Akai yūhi no Manshū ni – Or that I would dig a grave for my comrade
友の塚穴掘らうとは Tomo no tsukaana horō to wa – Under the red sunset of Manchuria
隈無く晴れた月今宵 Kumanaku hareta tsuki koyoi – This evening, under a perfectly clear moon
心染み染み筆執って Kokoro shimijimi fude totte – I take up the pen with the feeling of pity
友の最期を細々と Tomo no saigo wo komagoma to – And describe my comrade’s last moments
親御へ送る此の手紙 Oyago e okuru kono tegami – In this letter to his parents
筆の運びは拙いが Fude no hakobi wa tsutanai ga – While writing it with my poor handling of the pen
行燈の陰で親達の Andon no kage de oya-tachi no – I am deeply affected with the thought of the feeling his parents will experience
讀まるゝ心思ひ遣り Yomaruru kokoro omoiyari – When I read it by dim lamplight
思はず落とす一雫 Omowazu otosu hitoshizuku – I cannot stop a tear from falling
If you really wish to try to fit it in as battle music, here is a more upbeat version, and there are many different version you can listen to online:
https://www.youtube.com/watch?v=NWlv3USTIyM
EDIT:
Would also recommend the Japanese Army Review March and Warship March. Both marches lack lyrics, are big on brass, and were created during the late nineteenth century
https://www.youtube.com/watch?v=4viyTPzzzxI
At any rate, I'll stop hogging up the thread with videos now. I'd recommend the Fall of the Samurai Soundtrack, along with this channel which sports many Japanese military marches:
https://www.youtube.com/user/legendKuzorra/videos
u guys have planned to add marines from various european styled factions? Such as russians , with their famous stripped shirt and the cap, and also steamships 
