I'm not sure if being leftist necessarily means being pacifist. If I'm not mistaken, you're referring to Orwell's essay "The Lion and the Unicorn: Socialism and the English Genius" in which he calls for a kind of socialist English patriotism. This view may have been a minority view among the British left at the time, but it was by no means uncommon. There are a myriad of examples of leftist movements that have supported war: Bolshevism, German Social Democrats during World War I, the international communist movement after the Nazi invasion of the Soviet Union, etc. I could go on.

Your question veers away from the historical, but I'll try to answer it to the best that I can from a historical perspective. The left' has long been divided on the question of war and conflict. Perhaps the earliest iteration came in the mid-19th to early-20th century, when you had a split in the labor movement over the wisdom of strike actions versus revolutionary socialism. This divide became far more apparent during the Russian October Revolution. I suppose the more orthodox Marxists within different labor movements throughout Western Europe and within Russia could be viewed as "pacifists", although the Mensheviks in Russia opted to continue Russia's involvement in World War I (an action that, arguably, marked the point of no return for the impending Bolshevik Revolution). To the extent that I'm aware of, pacifism was not necessarily a widespread belief among socialist circles in the early twentieth century. Again, it could be argued that those social democrats and orthodox Marxists may have been the more moderate voices within the socialist movement, but they always accepted the inevitability of revolution, while also encouraging anti-imperialist struggles throughout the world.

The point where we begin to see pacifism as something associated with left-wing thinking occurs during the 1960s with the rise of the New Left. In part, the New Left was a reaction to Soviet imperialism in Eastern Europe, especially after the invasion of Hungary. Many leading intellectuals within communist parties throughout the world resigned their memberships in protest of that invasion (including the famous historian EP Thompson). However, the New Left was not simply an attempt to create a non-aligned leftist movement- it was also a reaction to the perceived threat of Western imperialism, provoked by events in Vietnam, Iran, and elsewhere. The New Left positioned itself as a kind of grassroots student movement opposed to imperial aggression. It also incorporated protest methods and philosophies of the US Civil Rights Movement, which was explicitly pacifist. Within a US context, the Democratic Party's support for the Vietnam war, the decline of the US labor movement, and the political shift to the right at the end of the 1960s all contributed to the severe curtailment of leftist politics within the United States. Those on the left who witnessed this decline therefore saw pacifism as a position that could be taken to set themselves apart from what was perceived to be a co-opted and bought American left.

To conclude, war and the use of force have been advocated by the left on plenty of occasions. The notion that the left is more in favor of pacifism is really more a product of the 1960s and would have been a rather alien concept for a socialist like Orwell.