a recruitment slogan for the United States Army that was made up by some Madison Avenue guys that helped drive volunteerism after the end of the Vietnam War and compulsory draft era. Went strong through the 80s and 90s, but was replaced at the turn of the century. They were so happy with this advertising slogan that they gave the guy who made it an Outstanding Civilian Service Award. Since 2001, slogans have included "Army of One" during the early W. Bush era, the Hulk-like "Army Strong" (they intended to use "army" as an adjective), and most recently, "Warriors Wanted".
Grammar aside: this phrase is interesting because it's wrong. My theory is that the Army slogan is a deliberately imperfect sentence as an advertising technique. It's "wrong" in a very small way, but because these slogans are carefully calculated, it's likely intentional. Correct grammar is professional, prosaic, easily ignored. By delivering a sentence that is not grammatically perfect, it resists logical satiation; the sentence does not fully resolve, and the reader gives it another pass. This is supported by the sheer number of people who incorrectly quote the sentence. There are three words missing from the middle: "be all of that which you can be". By omitting those words, the slogan connects the two phrases "be all" to "you can be" (both which could be read separately as stubs) to form a phrase with incorrect grammar.
The phrasing also directs the audience towards ideas about manifestation and the future. The affirmative "can be" is read as "can become", a future-facing context that is not verbally explicit, but which is the only way to parse the sentence. It incepts ideas about effort, potential and change. The empowering phrase "you can" is central to the slogan, sandwiched between an affirmative and an imperative. It then allows for the complete sentence "be all you can", before, again, resisting grammatical satiation with the second "be". Pronunciation (and singing in the commercials) typically puts stress on the "all" because of the syllabic distance between it and "you", emphasizing potential and magnitude. This slogan is semiotically clear and instantly catchy, and delivers multiple concepts via precisely designed imperfect grammar hooks.
The correct, cleaned up, standardized test version would look like this:
It's oddly mundane, when observing grammar rules.