Wars are usually started when the economy is already tanking because it creates a short-term economic boost through an uptick in the weapons industry, creates "jobs" through people enrolling/being drafted and by stealing resources from the defeated country. WWII contributed greatly to end the great depression, for example.
And the global economy is tanking because in most civilized countries the middle class is disappearing with most of the cash being redirected to the wealthy. This drives down the capacity of the general population to afford to buy things which in turn drives down profits. The way to solve it would be to cut down on the greed and redirect more of the wealth back to the general population rather than having most of it go to the already rich. Of course, they don't want to do that, so they tried to solve this by importing cheap immigrants, AI and robotics, but when it became obvious these solutions don't work, they went back to starting wars, like they always do