The Marxist approach to conflicts and wars is based on the understanding that they are not a separate issue of politics in general but rather a specific aspect of it. Friedrich Engels and V.I. Lenin were big admirers of the Prussian military theorist Carl von Clausewitz of the early 19th century, who summarized the essence of any military conflict by the famous words: “War is a mere continuation of policy by other means.” 
In his book On War Clausewitz stressed and elaborated this point: “War is an instrument of policy; it must necessarily bear its character, it must measure with its scale: the conduct of war, in its great features, is therefore policy itself, which takes up the sword in place of the pen, but does not on that account cease to think according to its own laws.” 
Indeed, wars are waged by any class not as an end in itself but for specific political and economic goals, i.e. in order to impose their interests. This applies both to the policy of the bourgeoisie as well as for the policy of the proletariat and other oppressed classes.
An imperialist state might attack a rival in order to get control over strategic territories or its colonies (e.g. World War I between Germany, France, Britain, Russia, et.), a Great Power might attack a semi-colony in order to rob its economic resources (e.g. the US invasion of Iraq 2003) or to stop a rebellion undermining the imperialist domination in the region (e.g. Russia’s two wars against Chechnya in the mid-1990s and early 2000s). Or it can be a war of the oppressed classes in order to drive out foreign invaders (e.g. the Afghan resistance against the U.S. forces since 2001 or the Somali struggle against the Ethiopian-led AU troops) or to bring down a reactionary dictatorship (e.g. in Cuba against Batista 1953-59, in Libya against Gaddafi in 2011 or in Syria against Assad since 2011).
To put it in the words of Lenin: „With reference to wars, the main thesis of dialectics, which has been so shamelessly distorted by Plekhanov to please the bourgeoisie, is that “war is simply the continuation of politics by other [i.e., violent] means”. Such is the formula of Clausewitz, one of the greatest writers on the history of war, whose thinking was stimulated by Hegel. And it was always the standpoint of Marx and Engels, who regarded any war as the continuation of the politics of the powers concerned— and the various classes within these countries—in a definite period.“ 
War is part of politics; the latter can not exist without the former. A pamphlet of the US-American Trotskyists published in 1936 formulated this fundamental idea with much apropos:
“The moral, religious, racial and ideological disguises that war wears must not be allowed to hide the fundamental conflicts which are the true sources of modern war. The general conclusion is inescapable: Modern war is neither accidental nor due to the evil of human nature nor decreed by God. War is of the very essence of imperialist-capitalism, as much a part of capitalism as wage labor. To speak of capitalism without war is like speaking of a human being without lungs. The fate of one is inextricably bound to the fate of the other.” 
From this follows that Marxists approach issues of wars not with a different method than other features of the struggles between classes. Hence, the working class policy is directed towards defending its independence from the ruling class (respectively its different factions) of all imperialist powers in times of peace as well as in times of war. And it is fighting in order to weaken the capitalists and eventually to overthrow them in times of peace as well as in times of war.
Lenin pointed to the fundamentally same principles of the class struggle in times of peace as well as during wars: “War is a continuation of policy by other means. All wars are inseparable from the political systems that engender them. The policy which a given state, a given class within that state, pursued for a long time before the war is inevitably continued by that same class during the war, the form of action alone being changed.” 
This idea was repeated by Trotsky in the founding program of the Fourth International: “Imperialist war is the continuation and sharpening of the predatory politics of the bourgeoisie. The struggle of the proletariat against war is the continuation and sharpening of its class struggle. The beginning of war alters the situation and partially the means of struggle between the classes, but not the aim and basic course.” 
This is true both for issues of domestic as well as foreign policy. Trotsky insisted in his theses on War and the Fourth International on the close and inseparable relationship between the internal and external policies of both the ruling class and the proletariat. The vanguard of the proletariat advocates a policy of class independence from any imperialist bourgeoisie and from each of their fractions – both of those at home and of those abroad: “The foreign policy of each class is the continuation and development of its internal policy.” 
In other words, the entire method of revolutionary defeatism has no "special tactics" for war, but rather dictates the continuation of tactics directed to promote the independence of the working class of every imperialist bourgeoisie (and each fraction of this), which are valid for all phases of the class struggle – whether in times of war or peace.
For this reason, Marxists apply the very same method of "revolutionary defeatism" not just in cases of conflicts between imperialist countries. Likewise the RCIT also applies such a method, for example, on issues like the membership of imperialist states in inter-state alliances (e.g. the issue of membership in the European Union). 
Another, similar, issue is the case of elections in which only open-bourgeois candidates are competing (e.g., in the presidential election in the USA between the candidates of the Republican and the Democratic Parties). In such situations, revolutionaries cannot support any of these candidates and therefore call for abstention. 
In this context it is worth drawing attention to Lenin’s observation that revolutionaries should study the writings of Clausewitz because of the closeness of political tactics and military tactics. In an article published during the last period of Lenin’s lifetime, the Soviet historian V.I. Sorin attributed to the leader of the Bolshevik the following views on the significance of Clausewitz : "Lenin said that 'political and military tactics are called Grenzgebiet (a borderland) in German and party workers could study with advantage the works of Clausewitz, the greatest of German military theoreticians.'" 
In summary, Marxists don’t change their approach to the ruling class in times of war, hence, the working class must fundamentally oppose its imperialist government equally in times of peace as well as in times of war. The politics of revolutionary defeatism has always the same programmatic root: namely, the struggle for the political independence of the working class from all fractions of the bourgeoisie and from all imperialist powers.
 Carl von Clausewitz: Vom Kriege (1832), Hamburg 1963, p. 22; in English: Carl von Clausewitz: On War, http://www.gutenberg.org/files/1946/1946-h/1946-h.htm
 Carl von Clausewitz: Vom Kriege (1832), Hamburg 1963, p. 221; in English: Carl von Clausewitz: On War, http://www.clausewitz.com/readings/OnWar1873/Bk8ch06.html#B
 V.I.Lenin: The Collapse of the Second International (1915), in: LCW Vol. 21, p.219 (Emphasis in the original)
 John West (James Burnham): War and the Workers (1936), Workers Party Pamphlet, https://www.marxists.org/history/etol/writers/burnham/1936/war/index.htm
 V. I. Lenin: War and Revolution (1917), in: LCW 24, p. 400
 Leon Trotsky: The Death Agony of Capitalism and the Tasks of the Fourth International. The Transitional Program (1938); in: Documents of the Fourth International, New York 1973, p. 199
 Leon Trotsky: War and the Fourth International (1934), in: Trotsky Writings 1933-34, p. 313
 See on this e.g. Michael Pröbsting: Marxism, the European Union and Brexit. The L5I and the European Union: A Right Turn away from Marxism. The recent change in the L5I’s position towards the support for EU membership represents a shift away from its own tradition, of the Marxist method, and of the facts; August 2016, in: Revolutionary Communist No. 55, http://www.thecommunists.net/theory/eu-and-brexit/; Michael Pröbsting: Does the EU Represent "Bourgeois Democratic Progress"? Once again, on the EU and the Tactics of the Working Class – An Addendum to our Criticism of the L5I’s Turn to the Right and Its Support for EU Membership, 16.09.2016, https://www.thecommunists.net/theory/eu-brexit-article/; RCIT: After the BREXIT Vote – Stormy times ahead for the workers and oppressed in Britain, 24.6.2016, http://www.thecommunists.net/worldwide/europe/brexit-vote-results/; RED*LIBERATION (Bulletin of Socialists in the Labour Party): UK: No to Cameron’s Trap: Neither YES nor NO to UK membership in the EU! For Abstention in the Referendum! We call on Momentum to create a “Third Camp” and to launch a socialist and internationalist campaign! For international Unity of the British, Migrant and European Workers! 25 February 2016, https://redliberation.wordpress.com/2016/05/02/100/; RCIT und RCIT Britain: Boycott Cameron’s Trap: Neither Brussels, nor Downing Street! For Abstention in Britain’s EU-Referendum! For international Unity and Struggle of the Workers and Oppressed! Fight against both British as well as European Imperialism! Forward to the United Socialist States of Europe, 2 August 2015, http://www.thecommunists.net/worldwide/europe/eu-referendum-in-uk/; Michael Pröbsting: The British Left and the EU-Referendum: The Many Faces of pro-UK or pro-EU Social-Imperialism. An analysis of the left’s failure to fight for an independent, internationalist and socialist stance both against British as well as European imperialism, Revolutionary Communism Nr. 40, August 2015 http://www.thecommunists.net/theory/british-left-and-eu-referendum/
 See on this e.g. Yossi Schwarz: Why Not to Vote for the Democratic Party in the Forthcoming US Elections Or At Any Other Time, 2.3.2016, https://www.thecommunists.net/worldwide/north-america/no-vote-sanders/; Yossi Schwartz: Once Again: Opportunism of US Left Exposed. An Analysis of the US 2016 Elections Campaign, 14 August 2016, https://www.thecommunists.net/worldwide/north-america/left-and-us-election/; Michael Pröbsting: The Meaning, Consequences and Lessons of Trump‘s Victory. On the Lessons of the US Presidential Election Outcome and the Perspectives for the Domestic and International Class Struggle, 24.November 2016, https://www.thecommunists.net/theory/meaning-of-trump/
 V.I.Sorin: Marxism, Tactics, and Lenin (Pravda, No. 1, 1923), quoted in A.S. Bubnov: Preface to Lenin's "Notebook on Clausewitz" (1931), in Donald E. Davis and Walter S.G. Kohn: Lenin on Clausewitz, in: Soviet Armed Forces Review Annual, Vol. I, Academic International Press, 1977, Gulf Breeze, Florida, p. 193. On Lenin’s thoughts on Clausewitz and the military question in general, in addition to his Notebook mentioned above, see also: Jacob W. Kipp: Lenin and Clausewitz: The Militarization of Marxism, 1914-1921, in: Military Affairs Vol. 49, 1985, pp. 184-191; James Ryan: ‘Revolution is War’: The Development of the Thought of V. I. Lenin on Violence, 1899–1907, in: The Slavonic and East European Review, Vol. 89, No. 2 (April 2011), pp. 248-273; Israel Getzler: Lenin's Conception of Revolution As Civil War, in: The Slavonic and East European Review, Vol. 74, No. 3 (Jul., 1996), pp. 464-472.
We venture to take this opportunity to inform readers about the following little anecdote which is very characteristic for the phony character of Stalinism. As the author of these lines is in possession of both the German as well as the English language publication of Lenin’s Notebook on Clausewitz, he could compare the prefaces of both editions. They are identical, word for word. However, while the original one was written by A.S. Bubnov in 1931, the German-language version has been published under the name “Otto Braun”. The reason is simply that Bubnov meanwhile has been persecuted by the Stalinists so the German Stalinist simply took his preface and published it under his own name! (The German language edition is: W.I.Lenin: Clausewitz’ Werk ‘Vom Kriege’. Auszüge und Randglossen, Verlag des Ministeriums für nationale Landesverteidgung, Berlin 1957)
We know that the Stalinists have implemented such a policy of suppression of original authors many times. For example, the “father” of the Collected Works of Marx and Engels was the great historian David Rjazanov. But since he was persecuted and later killed by the Stalinists, his name never appeared in the famous volumes! (On Rjazanov work on the Collected Works of Marx and Engels see e.g. David Borisovič Rjazanov: Vorwort zur MEGA 1927, in: UTOPIE kreativ, H. 206 (December 2007), pp. 1095-1011; Bud Burkhard: D. B. Rjazanov and the Marx-Engels Institute: Notes toward further Research, in: Studies in Soviet Thought 30 (1985), pp. 39-54; Colum Leckey: David Riazanov and Russian Marxism, in: Russian History/Histoire Russe, Vol. 22, N° 2 (1995); David Borisovič Rjazanov und die erste MEGA. Beiträge zur Marx-Engels-Forschung. Neue Folge. Sonderband 1. Argument, Hamburg 1996)