Nobody ever said it better than Abraham Lincoln in the Gettysburg Address:

"But, in a larger sense, we cannot dedicate—we cannot consecrate—we cannot hallow—this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. The world will little note, nor long remember what we say here, but it can never forget what they did here. It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us—that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under God, shall have a new birth of freedom— and that government of the people, by the people, for the people, shall not perish from the earth."

or "Those who *have* died in wars..."

a bit different.....

This is the phrase that automatically comes to mind, it's so well-known that it can take the place of the entire lengthy translation.
Comes from a poem by Rudyard Kipling entitled Recessional.
Used in Remembrance Day ceremonies as a caution against forgetting those who died in war.

See also
http://en.wiktionary.org/wiki/lest_we_forget

Those who gave their lives in the wars must serve as a reminder to the living of peace' but am unhappy with the impact and word order of the last part.

sugg:
The dead from wars past shall remind the living to keep the peace

This works nicely for my ear. I know "counsel" is not quite the same as mahnen, perhaps more benign, but I think the drama of "the dead of war" balances this.

"Those who died in war remind those who live in peace."

That's about as short and as close to the German as I can get it without getting tangled up in articles. One could substitute 'warn' for 'remind' but that would technically get a bit away from the original German. As in the original, I see no need to include any sort of 'remind of what' as it's obvious.

(I was under the impression that the sentence should go here and the target term should go above. However...please pardon the double submission!)