背水の陣 (はいすいのじん, haisuinojin) indicates that you’re facing a last stand and there is nowhere to retreat to. This idiom is very similar to the English expression “one’s back against the wall”, but 背水の陣 emphasizes that one has to pull out all the stops to overcome a challenging situation. Therefore, even if you use this word to refer to your undesirable situation, it sounds rather cool and positive.
This phrase is often used as “背水の陣で臨む”, meaning “tackle something 背水の陣で（by pulling off all the stops in the last stand）. Here, “臨む (のぞむ)” means “tackle” or “work on”.
背水の陣 consists of the two words “背水” and “陣” with one particle “の” between them. The first word contains the two kanji characters “背” meaning “back” and “水” meaning “water”, and indicates that one’s back is against the water. The latter word “陣” means “a group of soldiers”. Therefore, 背水の陣 illustrates that soldiers are in a last stand with their back against the water, pulling out all the stops as to fight against their opponents.
Indeed, there are a number of synonyms of 背水の陣. Some of the common but advanced phrases are described briefly as follows:
四面楚歌: a challenging situation where you’re surrounded by people who are all against you.
八方塞がり: a tough situation where there is an obstacle in every direction; predicament
絶体絶命: a hopeless and desperate situation; a last stand
万事休す: a hopeless situation where there is nothing you can do
Note that compared to 背水の陣, these phrase have much more negative connotations.
I tackled this work 背水の陣で (by pulling out all the stops in a last stand)
In order to win the league, we must achieve a victory in this match by any means. This is exactly 背水の陣 (a last stand).
Tanaka, who 背水の陣で臨んだ (faced the game in a last stand), scored the first victory in this season.
Synonyms: 四面楚歌, 八方塞がり, 絶体絶命, 万事休す (see above)
Synonyms in English: last stand, with one’s back against the wall, at the end of one’s rope
Related words and phrases: None