Of course you don't have to lie to win at Diplomacy, but even in an iterated game scenario, I believe that never lying is a suboptimal strategy. As the article notes, players who do this are derisively called "care bears" and in my experience, aren't the most successful, though they may come in second fairly often (speaking from experience as something of a care bear myself.)

Over multiple repeated games with the same players, no doubt some sort of equilibrium arises where on the scale of 0 to 1, zero always being a care bear, and one always being cutthroat, an optimal strategy probably lies... well, I'd be curious to know exactly where that optimum lies, actually. Anyone care to guess? My guess would be somewhere around ~.25.