Economical warfare never went away, there is a war going on in Syria right now that is literally fought by tens of nations, Turkey from one place, Iran from another, Russia, Saudi Arabia, three fronts inside with regime, rebels and ISIS so there is literally a million ways that war can go, but nothing like that will happen because of Oil wars.
It will be economical and it will take a lot more to bring down these nations than just oil prices going down. Is Russia and Saudis affected by this? Sure they are, however they are not idiots neither, while we were buying oil from them and they were getting richer, they invested all of their money into global companies in every nation, if one day all oil ends in Saudi Arabia and they do not make any profits from it, they will still have billions of dollars invested spread around the world.
The fact is that due to what is happening today in the oil market, the Russian Federation will suffer the most. Their oil, which is the most expensive to produce, will not be able to compete with oil from Saudi Arabia or the United Arab Emirates. The Russian government will have to look for other approaches to sell its oil, since their oil and gas are the only sources for filling the state budget of the country. Based on this, Russia can begin any military operations in those places where it sees fit, in order to obtain profitable dividends or conditions in order to force other countries to negotiate favorable terms of trade. Modern warfare is not only bloodshed, but also an argument for doing business. As an example, this is the United States and Iraq.