From Random House:

"The expression to see the elephant has several related senses, all of which have to do with the idea of an elephant as a remarkable thing that one could see, for example, in a carnival.

The earliest, first recorded in 1835, is 'to see or experience all that one can endure; to see enough'. This presumably stems from the notion that once you have seen an elephant, no other sight could be as interesting. This sense is found in a number of examples through the 1850s, but then drops from use.

The major sense of the phrase is the broad 'to gain worldly experience; learn a lesson; lose one's innocence; see remarkable sights'. This dates from the 1840s and was relatively common throughout the nineteenth century, particularly in the American West. A specific subsense is the military 'to see combat, especially for the first time', which also dates from the 1840s. The reference of this phrase, then, is not to the actual use of elephants in combat; it's a figurative extension of an earlier meaning.

The phrase to see the elephant is still occasionally found. It is sometimes recognized as a nineteenth-century phrase--particularly from the Civil War--and is used in historical writing. It is also sometimes used in modern examples--we have examples of the 'to see combat' sense referring to the Vietnam War--but it is not that common, and is sometimes considered old-fashioned. "

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