|Some shared perspectives|
War is the ultimate end of the modern state. All the forces that go to make up the modern state combine to drive its activity in the direction of warfare. On the other hand, war is readily becoming more and more destructive, and has now reached a point in its development where it cannot be waged at all, on any considerable scale, without involving the destruction of civilization over the entire field of conflict. . . . [T]he traditional politics which in England is called democracy, and on the Continent liberalism, is here out of date. It thinks of war as an instrument which statesman are free to use or not to use in pursuit of their ends, whereas it is in reality a monster which, having invoked it, they not cannot exorcise. What began as a means to an end beyond itself has lost that character: it has become a thing that must be used, whatever comes of it. . . . Much of what has happened in militarist countries within the last few years suggests that in those countries what we call civilization is no longer valued. Freedom of thought and speech, personal liberty, and many other features of what we should call civilized life, have been deliberately repudiated with the avowed aim of rendering the nation a more docile and responsive fighting-machine. For the militarist, the incompatibility of civilization and war is only a nail in the coffin of civilization. The only corporate activity which he recognizes as desirable in a nation is warfare itself. 
"War is the ultimate end of the modern state." My first reaction is to consider this an overstatement. The modern state, in addition to its more traditional functions (law enforcement, provision of public goods) has added economic and social welfare to its portfolio. And, of course, the state has always been involved in defense of territory (and often it seeks the addition of territory). But in my life time (from near the beginning of the Cold War), the military function has grown remarkably in the U.S., whereas before WWII, the military had been surprisingly limited in its role. We Americans began to refer to the president as our "commander-in-chief," which he isn't unless we're members of the active duty military. All of this demonstrates the indisputable fact that war and democracy are inimical to one another. War will inevitably seek to strangle democracy as a threat to its powers. Democracy as an expression of the will of most people will seek to avoid war. (N.B. This last contention needs qualification. I believe that avoidance of war is a popular default position among non-elites; however, this default position of the demos is easily transposed into supporting war by propaganda appealing to nationalist or retaliatory sentiment.)
In reviewing the latter part of the quote, I noted how Collingwood anticipates Orwell, who published 1984 in 1949, seven years after Collingwood's death and 13 years after Collingwood penned this essay. Recall Oceania's perpetual state of war with Eurasia and Eastasia, each mega-state controlled by an ideology that extols and supports war.
Think also of Stephen Bannon and some of the thinkers whom he has favorably cited, such as the Italian fascist Julius Evola. These and other thinkers, for instance, Georges Sorel in fin de siecle France, argue that violence and war are a necessary tonic for peoples and nations. Indeed, WWI began with many on both sides of the fight believing that a war would alleviate the malaise felt in many parts of European society during the period leading up to the war. One would think such notions would have perished in the ashes and graveyards that the war created, and while this was true of many, a committed few concluded that an increase of dosage was required. Mussolini and Hitler are only the two most well known of that latter sort. I have not doubt that Collingwood was thinking of these two as he wrote this about the "militarists." And it's not difficult today to identify those who hold this militarist attitude.