Since 2008 we live in a historic revolutionary period caused by the tendency of the rate of profit of capitalist production to fall. This tendency leads to capitalist governments serving the very rich to attack economic social and democratic rights of the workers and the poor.
The capitalists in some countries like Germany are in crisis, in other countries right wing tendencies are on the rise. This crisis is reflected in the US in the form of Trump. This is also a period that the mass movement of the oppressed is on the rise. The current latest cases are Palestine and Iran.
The crisis of humanity in this period is the lack of strong revolutionary International leading socialist revolutions. The tendency of the rate of profit to fall is leading to trade wars. Unless we will overthrow the world capitalist system, the conflicts between the old imperialist states the US, Western Europe and Japan and the new imperialist states China and Russia, will lead to a third world war.
US President Donald Trump is already pushing for a trade war with China. He acts to restrict imported goods made by China: "The trade imbalance between America and China is one of Trump's signature moves, and a tariff is one way to address that kind of problem: It's a tax levied on imports from a particular country in order to make those imports more expensive. The U.S. trade deficit with China was a whopping $342.6 billion in 2014 — not $505 billion as Trump implied, which was America's trade deficit with the rest of the world combined. But that's still a problem. It means China sold $342.6 billion more worth of goods and services to America than America sold to China.” 
Wars between imperialist states, no matter how the capitalists and their servants portray themselves and what color they paint their faces with, are all about control over raw materials, markets and cheap labor. Right now, the competition between the new and the old imperialist states is intensifying.
WWI, which cost humanity about 20 million deaths, broke out because different imperialists wanted to re-divide the colonies. WWII, where the death toll almost tripled, broke out as the only way out of an economic crisis that began in 1929. Of course, the defeat of the revolutionary struggle of the working class contributed to humanity’s inability to stop it on time. This was mainly due to the counter-revolutionary policies of the social democrats, the Stalinists and centrists such as the POUM in Spain.
Both world wars destroyed a large portion of the forces of productions in Europe, Asia and Africa. They claimed the lives of about 100 million people, most of whom where civilians. Thus we live in a world where a mentally ill serial killer will be put to death or imprisoned for life, while those who are responsible for the deaths of millions will die of old age.
The very need of rebuilding the destroyed economy was behind the first-world prosperity after WWII. The capitalist world system does not present any way out of the vicious cycle of economic crisis except the destruction of large portions of the forces of production. A third world war will lead humanity back to the Stone Age and might even cause the total elimination of the human race. As Lenin put it: "For humanity to live imperialism must die."
This compels us to examine the question of wars and revolution. Marxists are not pacifists who oppose all wars. Marxists use the concept of the nineteenth century Prussian writer and soldier, Carl von Clausewitz, who wrote "war is the continuation of politics by other means."
Bourgeois peacetime politics is consisted of the exploitation of the working class, oppression of the working and non-capitalist women, exacerbation of racism and national chauvinism against immigrants (current scapegoats happen to be brown skinned or Muslim), destruction of the environment and, of course, preparing for a new global war. Lenin wrote that we have to look at "the class character of war: what caused that war, what classes are waging it, and what historical-economic conditions gave rise to it."
We revolutionary Marxists openly declare that our aim is to achieve a socialist society, which, by eliminating the division of mankind into classes, will inevitably eliminate the very possibility of war. However, on the road to a classless society we support wars waged by revolutionary classes, wars which are of direct and immediate revolutionary significance. Wars that advance the struggle of the working class and the oppressed against the imperialist enemy and its servants.
As Trotsky wrote: "We do not and never have put all wars on the same plane. Marx and Engels supported the revolutionary struggle of the Irish against Great Britain, of the Poles against the tsar, even though in these two nationalist wars the leaders were, for the most part, members of the bourgeoisie and even at times of the feudal aristocracy ... at all events, Catholic reactionaries. When Abdel-Krim rose up against France, the democrats and Social Democrats spoke with hate of the struggle of a "savage tyrant" against the "democracy." The party of Leon Blum supported this point of view. But we, Marxists and Bolsheviks, considered the struggle of the Riffians against imperialist domination as a progressive war. Lenin wrote hundreds of pages demonstrating the primary necessity of distinguishing between imperialist nations and the colonial and semi-colonial nations which comprise the great majority of humanity. To speak of "revolutionary defeatism" in general, without distinguishing between exploiter and exploited countries, is to make a miserable caricature of Bolshevism and to put that caricature at the service of the imperialists.” 
Trotsky explained why Marxists defend the struggle of the oppressed people when they are led by non-revolutionary and even reactionary leadership: "I will take the most simple and obvious example. In Brazil there now reigns a semi fascist regime that every revolutionary can only view with hatred. Let us assume, however, that on the morrow England enters into a military conflict with Brazil. I ask you on whose side of the conflict will the working class be? I will answer for myself personally—in this case I will be on the side of "fascist" Brazil against "democratic" Great Britain. Why? Because in the conflict between them it will not be a question of democracy or fascism. If England should be victorious, she will put another fascist in Rio de Janeiro and will place double chains on Brazil. If Brazil on the contrary should be victorious, it will give a mighty impulse to national and democratic consciousness of the country and will lead to the overthrow of the Vargas dictatorship. The defeat of England will at the same time deliver a blow to British imperialism and will give an impulse to the revolutionary movement of the British proletariat. Truly, one must have an empty head to reduce world antagonisms and military conflicts to the struggle between fascism and democracy. Under all masks one must know how to distinguish exploiters, slave-owners, and robbers!"