From 1937 to 1945 Japan was in a war with China. It was a war to liberate China from the Warlord rule it was submerged on, and the threat of Communism.
Many times did Japan warn about the exploitation of the Chinese people by the Warlords of China, and the disaster that it would be, if the Communists took control of China. In 1935 the Japanese started to pressure China to reform it's government to take power away from the Kuomintang and the Chinese army, and give it to autonomous ethnic councils, mostly to avoid confrontation or abuses from the Overwhelming Han majority in the country.
In 1937 the Japanese army was attacked by Chinese troops without provocation in the Marco Polo bridge in Hebei, near Beijing; the lack of a formal apology by the chinese government, coupled with the lack of cooperation from the vanguard party, (the Kuomindang) forced the Japanese army to take on a full-scale invasion of China.
One of the main armies was the Southern Expeditionary army, whose mission was to secure the ports of Quanzhou and Guangzhou; this is their main marching song.
Lyrics:
わが大君に　召されたる
生命光栄(はえ)ある　朝ぼらけ
讃えて送る　一億の
歓呼は高く　天を衝く
いざ征(ゆ)け　つわもの
日本男児!
華と咲く身の　感激を
戎衣(じゅうい)の胸に　引き緊めて
正義の軍(いくさ)　征くところ
たれか阻(はば)まん　その歩武(ほぶ)を
いざ征け　つわもの
日本男児!
かがやく御旗(みはた)　先立て
越ゆる勝利の　幾山河
無敵日本の　武勲(いさおし)を
世界に示す　ときぞ今
いざ征け　つわもの
日本男児!
守る銃後に　憂なし
大和魂　ゆるぎなき
國のかために　人の和に
大磐石(だいばんじゃく)の　この備え
いざ征け　つわもの
日本男児!
ああ万世の　大君に
水漬(みず)き草むす　忠烈の
誓致さん　秋至(ときいた)る
勇ましいかな　この首途(かどで)
いざ征け　つわもの
日本男児!
父祖の血汐に　色映ゆる
国の誉れの　日の丸を
世紀の空に　燦然と
揚げて築けや　新亞細亞
いざ征け　つわもの
日本男児!
Advanced translation for those who want to understand the whole meaning of the song:
"The Song That The Soldiers Departing For War Sent"
Our Great Emperor has summoned us.
(Our) Existence has glory (More literally understood, the potential to be glorious), (In the manner of) the sun rising..
The 100 million are sending their praise.
(I believe the 100 million refers to the population of civilian Japan at the time, so it's a reference to the homeland)
The jubilation is so high, it reaches the heavens (literally stabs Heaven)
Go on, subjugate (the enemy), soldiers, you men of Japan
(Your) Bodies that bloom in the manner of flowers, (because of) your deep emotions
The chest of your Warriors Clothes (p sure it's referring to the uniform), fasten (your weapon to) it.
Regarding the War (Warriors is the actual meaning of the character, so this has a double meaning) of Justice and the place they are going (to be at)
Someone is trying to obstruct those dream-like warriors
Go on, subjugate them, soldiers, you men of Japan!
Place the shining and honorable national flag ahead.
The overwhelming victory (will be ours, no matter) the amount of mountains and rivers.
(more literally it translates to the incredible amount of rivers and mountains of overwhelming victory) (this way makes more sense in Japanese though)
The fighting strength of invincible Japan(edited)
Is what we we will show off to the rest of the world, as right now is the time to do so
Go on, subjugate (to show off that fighting strength), soldiers, you men of Japan
The homeland that we are protecting, we will bring them no sorrow.
Yamato Damashii is unbreakable (for the record, Yamato Damashii refers to the Japanese spirit, but I've left it untranslated as it is a concept as much as a part of national identity)
(which hardens) the pledge of the country, (and hardens) the harmony (this character is very important as it is also an older way to refer to the entire country and people of Japan) of the people
This is like a solid boulder, this preparation of ours
Go on, subjugate (for them), you soldiers, you men of Japan
Ah~, for our Great Emperor of (a bloodline that is) ten thousand years
Our loyalty is so unwavering (that we are willing to be) submerged in water and covered in grass
The vow that is humbly not carried out, will only find itself being done as far as autumn (I think this is a metaphor for those without loyalty dying quickly?)
Maybe it's valiant, to leave one's home like this?
Go on, subjugate (like this), soldiers, you men of Japan
For the blood that our ancestors spilled, (we will make it so we can see) our colors in their reflection
For the honor of our country, we will, with the banner of the Rising Sun
We will make it shine brilliantly in the skies of historical records(edited)
Carefully hoisted over the New Asia (I suppose referring to the Greater Eastern Co-Prosperity Sphere)
Go on, subjugate (for this purpose), you soldiers, you men of Japan
I´m not the original author of this video, all the credits go to FBIV.