Catchy slogan. It's good, since no other currency can currently say it. But of course, if Bitcoin is mass adopted, war and violence will start to occur. Nice slogan, though.
How? War is incredibly expensive. If governments cannot inflate currency to fund wars their ability to launch them is limited. Thre is no way to finance a major war in a bit coin universe.
Countries use debt to finance wars. This is not the same as inflating their currency (using lower then market interest rates, QE, and the like).
There is little to no reason why a country couldn't issue debt denominated in bitcoin, much like other countries issue debt issued in US dollars.