You can open any newspaper on any given day and the top headlines will involve a war and how many people have died as a result of that war.
For thousands of years, people have become numb to the idea of war. From the top military analysts on down, they believe war will always exist. However, cancer has been around since the days of war, but no one believes that we cannot eradicate cancer some day.
Sun Tzu believed that, much like cancer, war is a destructive abnormality. It is an imbalance to the normal way of the world. His objective throughout The Art of War is to put the nation back into balance, back to stability. To say that war will always exist means to give into the notion that this imbalance is in fact normal, which it is not.
Furthermore, a society cannot prosper through destruction. Civilization cannot remain alive and vibrant if it cannot see the sun through the smoke of bombs and gunfire. War isn't the right answer because it asks the obvious wrong question: If you hurt another human being, wouldn't your situation be better?
The root cause of war is anger. Sun Tzu said anger can easily revert back to joy, but the dead cannot be brought back to life. So why would anyone want to engage in war and reap the permanent results of war when the actual ephemeral problem is vastly easier to resolve?
Therefore, according to Sun Tzu, to succeed at war is to succeed at preventing that war from happening in the first place. Such success in fact happens every day. The only news we read about in the newspapers are the news of failure. Entropy may be normal in nature, but with people involved, failures are not natural because nobody actively seeks to fail. People act to advance their interests, even in some small way. As such, failure is something you can prevent with proper planning and smart strategy as prescribed in The Art of War.
War is indeed a matter of life and death. Living fully is trying to shape the world we live in, to do what we feel is right, to do what must be done. To not do anything is quite the same action and produces the same result as a living death.