That depends on what you mean by “the” economy. If we are talking about the global economy, it is pretty easy to determine that the answer is no. War is not good for the economy. If there were no war, all the energy that goes into armies, navies, air forces, weapons, military bases, and etc. could be put into housing, food, clothing, medicine, entertainment, and etc. All the people and things that are destroyed by war could be put to more productive use. Eliminating war may be an impossible dream, but that is no reason not to work for a world with less war, smaller governments, smaller armies, and fewer prisons.
The sociological and psychological basics of war are the same as for any other criminal enterprise. When we ask if war is good for the economy, we are really asking if by going into a specific war, we can gain something that will more that cover the cost of the war. We know, or we should know, that the aggregate wealth of planet earth is going to be diminished by any war. What we are proposing when we think of initiating a war for profit is to increase our own wealth at the expense of others. We are proposing to enter into robbery and murder for profit.
In general, crime doesn't pay. It may pay for a while, but as a long term strategy, crime, and war are generally not good survival strategies. We can see examples of this in the animal kingdom. The lion is the quintessential career criminal. Some call him the king of the beasts, but to steal a phrase, the life of the lion is nasty, brutish, and short. On the other hand, the elephant enjoys a long and relatively tranquil life. Most large predators are having trouble surviving in the modern world, including the American eagle, but their prey prospers. The eagle is endangered, but there is no shortage of rabbits.
A criminal might not suffer for any particular crime, but eventually, he will pay dearly. An aggressive and authoritarian government may prosper for a while, but eventually, it will accumulate enough enemies to make it fall.
Often a war starts when the elite that control a society believe that they will benefit from the war. Most often they don't particularly care whether the war benefits the whole society, and it most often doesn't. The elite may believe that what is good for them is good for their society. They probably won't examine the question closely. Many rich and powerful people are better at manipulating social institutions for their own benefit than thinking about what is good for all the people in the institution. What is good for society as a whole may not be a top item on their priority list.
Popular opinion has it that World War II was responsible for the recovery of the American economy after the great depression. This myth has been thoroughly debunked by many extremely competent economists, but even if you still believe that WW II was good for the American economy, do you believe it was good for the world economy? If a rising tide lifts all boats, a perfect storm like WW II is a disaster for everyone. I would hate to think that we live in a society so immoral and ignorant that we would wreck the world on a slim chance that we could improve our economy. It is far easier to work for a living.
It strikes me as extremely peculiar that the United States would spend a hundred billion dollars or more and kill lots of people in a war on Iraq based on evidence that wouldn't be admissible in an ordinary burglary trial. Shock and awe seem to me to be a much bigger and smokier gun than any of the so-called evidence that has so far been presented.
Some say that the president knows things we don't know. He works for us. If he knows things we don't, and he won't tell, he should be fired.
What do we mean by “the" economy. When someone grows a tomato, and then eats it, that is economic activity, and a contribution to the Gross National Product, but that tomato will not get into the economic statistics. Economists don't think it exists. Especially government economists don't think such private activity is significant, or view it as something to be minimized or eliminated. Governments hate this kind of private, unregistered activity because they can't tax it, regulate it, or use it as an excuse to create a bureaucracy.
WW II caused women to enter the workforce in great numbers. The government loved that. The paycheck is by far the easiest target for taxation. The homegrown tomato is virtually un-taxable. When a woman makes her husband a shirt, the government doesn't get a cut. There was even a law against women sewing at home for profit. Now most citizens spend their entire lives in some institution or other. They may get a few years to enjoy their homes full time after they retire and before they enter a nursing home: Their lives will be defined by head start, kindergarten, grade school, high school, college, retirement and nursing home. Many people own four-bedroom house with three bathrooms, but spend most of their waking hours in an 8 by 10 cubicle sharing a communal bathroom with hundreds of strangers. We have declining birth rates, rising divorce rates, increasing incidence of serious mental health problems, record numbers of people in prison, homeless veterans of unnecessary wars on our streets, and an enormous tax burden. These are the true costs of war, and war is no bargain.
The economy has appeared to grow because fewer people are growing tomatoes and more people are working in factories. After WW II we have achieved the appearance of prosperity by nationalizing women and children. The government is a parasite. We are struggling harder and harder to feed the government tapeworm in the bowels of our society, and falling farther and farther behind.
Ask not what your government can do for you. Ask not what you can do for your government. Ask your government to take a hike.