I learned my first two Japanese terms for military ranks from watching The Adventures of Rin Tin Tin on Japanese TV during the 1950s. In the dubbed Japanese dialogue, the lieutenant was addressed as 中尉 chuui and the sergeant as 軍曹 gunsou. But I never learned the terms for the lowest ranks until I recently watched an epic film trilogy from the same era, The Human Condition (人間の條件, Ningen no jouken) via Netflix. It is very long and often slow-paced, but fascinating for both historical and linguistic reasons. It starkly depicts both the brutality of Japan's occupation of Manchuria and the deadly and chaotic aftermath of Japan's defeat there. Based on a novel, it also reflects the personal experience of the director, Kobayashi Masaaki (小林 正樹), a university-educated pacifist who refused to become an officer when drafted to serve in Manchuria.
The film chronicles the gradual erosion of socialist ideals in the face of insurmountable realities. The lead character first fails to transform a Japanese mine employing Chinese slave labor into a more humane and efficient enterprise. After being drafted for insubordination and joining a labor battalion, where he takes many a beating without fighting back, he is eventually forced to fire his weapon and kill an enemy soldier when Soviet tanks overrun his hapless platoon. As a prisoner of war, he finds that life under communism falls far short of the egalitarian paradise that he had imagined when he had earlier considered defecting. The Soviets treat him just as brutally as the Japanese imperialists treated their slave laborers. After he escapes, he ends up becoming a leader despite his low rank, forced to make life or death decisions about the fate of starving Japanese soldiers and colonists straggling back toward their homeland.
一等兵 ittouhei 'Private, PV2' – (Japanese Wikipedia offers the most thorough compilation of terms for military ranks in multiple languages that I have found so far.) The rank just below 一等兵 ittouhei (lit. '1st-level solider') 'PV2' is 二等兵 nitouhei (lit. '2nd-level soldier') 'Private, PV1' and the rank just above it is 上等兵 joutouhei (lit. 'upper-level soldier') 'Private First Class, PFC'.
When I faced the draft after dropping out of college in 1969, I had rather pacifist tendencies, which were fortunately never tested in real conflict. I opted for language school rather than Officer Candidate School, and never even had to fire a weapon after basic training. As company clerk, I would just qualify myself on paper. By the time I got out in 1972, I had reached the rank of SP5, a rank abolished in 1985 that corresponds the lowest level of SGT.
敗北 haiboku (lit. 'lose-north') 'defeat, rout' – The Human Condition (人間の條件, Ningen no jouken) set of DVDs from Netflix contains 3 interviews: one not very remarkable one with the director, Kobayashi; a much more recent and interesting one with the star, Nakadai Tetsuya (仲代 達矢), who many years later starred in Kurosawa's classic Ran; and a truly excellent retrospective with Shinoda Masahiro (篠田 正浩), who puts the trilogy in much broader context.
Shinoda uses a lot of contemporary gairai-go, but the word he uses for Japan's defeat is 敗北 haiboku (lit. 'lose-north'), a word that goes back to the Heike Monogatari, about the epic struggle for supremacy in 12th-century Japan between two clans, the Taira (or Heike) and Minamoto (Genji). One of its synonyms is 敗走 haisou ('lose-run').
So what does 北 'north' have to do with fleeing a lost battlefield? Does it suggest retreating to the northern frontier of Heian Japan, that is, northern Honshu? Or does it suggest losing the north at your back, as the imperial palaces were oriented in Kyoto, Seoul, Beijing, Xian, and other capitals within the Sinosphere? In modern Mandarin, the term 败北 bàiběi ('lose-north') is literary, implying it goes back a long way and was not adopted from Japanese (as many modern coinages were). The more common way to write 'defeat' in Chinese is 打败 dǎbài ('hit-lose').
UPDATE: Matt of No-sword has a few observations about the Japanese association of 北 'north' with flight from battle and with death.