Why war can remit the economic crisis？ In the history, like Japan, in the World War II, they had a hand in the war, the root cause is the economic depression.

And in this century, some country use this method many times to remit the economic problem.

So, why war can remit the economic crisis, I just don't understand?
It is not war itself that helps to counteract economic crises, but rather the increased spending needed to support the war. In terms of the evidence to support this claim, Lee Hudson Teslik from the Council on Foreign Relations writes:


  There is an ongoing debate about the extent to which war spending affects a country’s economy. Experts disagree on the most fundamental point—whether war helps or hurts national economic prospects. Massive U.S. national defense spending during World War II is sometimes credited with rejuvenating U.S. economic prospects following the Great Depression. The journalist Robert J. Samuelson, in a primer on the topic, says there can be little doubt that military spending and mobilization during World War II reduced U.S. unemployment rates and revitalized the economy. A recent paper from the National Bureau of Economic Research concludes that countries with high military expenditures during World War II showed strong economic growth following the war, but says this growth can be credited more to population growth than war spending. The paper finds that war spending had only minimal effects on per-capita economic activity. The effects of more recent wars are equally disputed. A historical survey of the U.S. economy from the U.S. State Department reports the Vietnam War had a mixed economic impact. The first Gulf War typically meets criticism for having pushed the United States toward a 1991 recession. In a 2003 op-ed in the Guardian, economist [Joseph] Stiglitz wrote that the aftermath of the Gulf War exposes the “myth of the war economy.” Indeed, he argues that increased military spending is “unambiguously bad” for the living standards of normal citizens. Other economists argue the opposite.


I would argue the consensus generally leans towards wartime spending as beneficial to a nation's economy.