An emergency call has come through on the flashing red telephone I keep handy in case anybody has any obscure questions they are unable to google.
"Finn! Can you please explain: "Soldiers soldiers soldiers fight fight fight."?
I don't actually have a flashing red telephone, but I do have a facebook account and I am always pleased to receive your obscure questions as it saves me having to think up something to write about. The question is beautifully googleproof so, even though I started to answer it on my facebook page, I thought it deserved a fuller treatment here for posterity.
The sentence, "Soldiers soldiers soldiers fight fight fight", has been devised by wily linguists to show the limitations of the human mind's ability to parse overly recursive sentences.
A couple of technical definitions: To parse is, broadly speaking, to read and make sense of a sentence. Recursion is the property of language that allows a category to include an instance of itself. For example, a sentence may consist of two sentences linked by "if... then...". A verb phrase can be constructed from a verb and a noun phrase followed by a verb phrase. (Irrelevantly, the verb phrase, "...can be constructed from a verb and a noun phrase followed by a verb phrase," also happens to be an example of itself - but that's just me showing off.)
Back to our main plot:
"Soldiers soldiers soldiers fight fight fight" is an example of a triple embedded sentence. Triply embedded sentences are on the borderline of parsability. We can't understand them but we can understand how they would be understood if our brains were capable of it.
Let's break it down.
"Soldiers fight" is a sentence any native speaker can parse.
"Soldiers that soldiers fight also fight" is a similarly understandable sentence.
In English, the "that" and the "also" are optional. We can lose them and have the double embedded sentence, "Soldiers soldiers fight fight". This sentence works in the same way as "Doors I close stay shut", or "Girls Georgie-Porgie kisses cry." It is a bit more confusing because both nouns are the same and one of the verbs is transitive and the other intransitive, but we can pretty much work out which verb belongs to which regiment and get a pretty clear picture of what is going on.
Now, if "Soldiers soldiers fight fight" is a legitimate sentence then we should be able to use the principle of recursion to plonk it down in the middle of "Soldiers fight" and give us our target sentence of "Soldiers soldiers soldiers fight fight fight". In fact we should be able to use the principle of recursion to extend our "soldiers" and our "fight" out to infinity.
Our human brains are capable of understanding that principle, but incapable of parsing a triple embedded sentence.