When I was 10 years old, the 26th Amendment to the US Constitution extended the vote to everyone over 18 years old. The main argument for this extension of the franchise was that someone old enough to die for his country was old enough to vote. (The argument focused on young men who were being drafted, but young women got the vote, too.) This association of military service and the right to vote goes way back;, as John Ferejohn and Frances McCall Rosenbluth's book "Forged Through Fire: War, Peace, and the Democratic Bargain" argues.  Elites need soldiers to fight fiercely in order to preserve national autonomy. Letting soldiers vote helps in that. 

John Locke points to the converse in Sections 107-110 of his 2d Treatise on Government: “Of Civil Government” (in Chapter VIII, "Of the Beginning of Political Societies"). People under threat know they need a war leader. The war leader, often called a king, therefore often has the consent of the governed for relatively autocratic rule in the conduct of war. But John Locke argues that this selection of a war leader did not confer on the war leader autocratic power in other domains. Of course a successful war leader could could often seize autocratic power in other domains, but that power did not have the same legitimacy as the autocratic power over the conduct of war. I have included a lead-in to Sections 107-110 at the end of Section 106: 

§. 106. ... all petty monarchies, that is, almost all monarchies, near their original, have been commonly, at least upon occasion, elective.

 §. 107. First then, in the beginning of things, the father’s government of the childhood of those sprung from him, having accustomed them to the rule of one man, and taught them that where it was exercised with care and skill, with affection and love to those under it, it was sufficient to procure and preserve to men all the political happiness they sought for in society. It was no wonder that they should pitch upon, and naturally run into that form of government, which from their infancy they had been all accustomed to; and which, by experience, they had found both easy and safe. To which, if we add, that monarchy being simple, and most obvious to men, whom neither experience had instructed in forms of government, nor the ambition or insolence of empire had taught to beware of the encroachments of prerogative, or the inconveniences of absolute power, which monarchy in succession was apt to lay claim to, and bring upon them; it was not at all strange, that they should not much trouble themselves to think of methods of restraining any exorbitances of those to whom they had given the authority over them, and of balancing the power of government, by placing several parts of it in different hands. They had neither felt the oppression of tyrannical dominion, nor did the fashion of the age, nor their possessions, or way of living, (which afforded little matter for covetousness or ambition) give them any reason to apprehend or provide against it: and therefore it is no wonder they put themselves into such a frame of government, as was not only, as I said, most obvious and simple, but also best suited to their present state and condition; which stood more in need of defence against foreign invasions and injuries, than of multiplicity of laws. The equality of a simple poor way of living, confining their desires within the narrow bounds of each man’s small property, made few controversies, and so no need of many laws to decide them, or variety of officers to superintend the process, or look after the execution of justice, where there were but few trespasses, and few offenders. Since then those, who liked one another so well as to join into society, cannot but be supposed to have some acquaintance and friendship together, and some trust one in another; they could not but have greater apprehensions of others, than of one another: and therefore their first care and thought cannot but be supposed to be, how to secure themselves against foreign force. It was natural for them to put themselves under a frame of government which might best serve to that end, and chuse the wisest and bravest man to conduct them in their wars, and lead them out against their enemies, and in this chiefly be their ruler.

  §. 108. Thus we see, that the kings of the Indians in America, which is still a pattern of the first ages in Asia and Europe, whilst the inhabitants were too few for the country, and want of people and money gave men no temptation to enlarge their possessions of land, or contest for wider extent of ground, are little more than generals of their armies; and though they command absolutely in war, yet at home and in time of peace they exercise very little dominion, and have but a very moderate sovereignty, the resolutions of peace and war being ordinarily either in the people, or in a council. Though the war itself, which admits not of plurality of governors, naturally devolves the command into the king’s sole authority.

  §. 109. And thus in Israel itself, the chief business of their judges, and first kings, seems to have been to be captains in war, and leaders of their armies; which (besides what is signified by going out and in before the people, which was to march forth to war, and home again in the heads of their forces) appears plainly in the story of Jephtha. The Ammonites making war upon Israel, the Gileadites in fear send to Jephtha, a bastard of their family whom they had cast off, and article with him, if he will assist them against the Ammonites, to make him their ruler; which they do in these words, “And the people made him head and captain over them,” Judges xi. 11. which was, as it seems, all one as to be judge. “And he judged Israel,” Judges xii. 7. that is, was their captain-general six years. So when Jotham upbraids the Shechemites with the obligation they had to Gideon, who had been their judge and ruler, he tells them, “He fought for you, and adventured his life far, and delivered you out of the hands of Midian,” Judg. ix. 17. Nothing mentioned of him, but what he did as a general: and indeed that is all is found in his history, or in any of the rest of the judges. And Abimelech particularly is called king, though at most he was but their general. And when, being weary of the ill conduct of Samuel’s sons, the children of Israel desired a king, like all the nations to judge them, and to go out before them, and to fight their battles, 1 Sam. viii. 20. God granting their desire, says to Samuel, “I will send thee a man, and thou shalt anoint him to be captain over my people Israel, that he may save my people out of the hands of the Philistines,” ix. 16. As if the only business of a king had been to lead out their armies, and fight in their defence; and accordingly at his inauguration pouring a vial of oil upon him, declares to Saul, that the Lord had anointed him to be captain over his inheritance, x. 1. And therefore those, who after Saul’s being solemnly chosen and saluted king by the tribes at Mispah, were unwilling to have him their king, made no other objection but this, “How shall this man save us?” v. 27. as if they should have said, this man is unfit to be our king, not having skill and conduct enough in war, to be able to defend us. And when God resolved to transfer the government to David, it is in these words, “But now thy kingdom shall not continue: the Lord hath sought him a man after his own heart, and the Lord hath commanded him to be captain over his people,” xiii. 14. As if the whole kingly authority were nothing else but to be their general: and therefore the tribes who had stuck to Saul’s family, and opposed David’s reign, when they came to Hebron with terms of submission to him, they tell him, amongst other arguments they had to submit to him as to their king, that he was in effect their king in Saul’s time, and therefore they had no reason but to receive him as their king now. “Also (say they) in time past, when Saul was king over us, thou wast he that leddest out and broughtest in Israel, and the Lord said unto thee, Thou shalt feed my people Israel, and thou shalt be a captain over Israel.”

  §. 110. Thus, whether a family by degrees grew up into a commonwealth, and the fatherly authority being continued on to the elder son, every one in his turn growing up under it, tacitly submitted to it, and the easiness and equality of it not offending any one, every one acquiesced, till time seemed to have confirmed it, and settled a right of succession by prescription: or whether several families, or the descendents of several families, whom chance, neighbourhood, or business brought together, uniting into society, the need of a general, whose conduct might defend them against their enemies in war, and the great confidence the innocence and sincerity of that poor but virtuous age, (such as are almost all those which begin governments, that ever come to last in the world) gave men one of another, made the first beginners of commonwealths generally put the rule into one man’s hand, without any other express limitation or restraint, but what the nature of the thing, and the end of government required: which ever of those it was that at first put the rule into the hands of a single person, certain it is no body was intrusted with it but for the public good and safety, and to those ends, in the infancies of commonwealths, those who had it commonly used it. And unless they had done so, young societies could not have subsisted; without such nursing fathers tender and careful of the public weal, all governments would have sunk under the weakness and infirmities of their infancy, and the prince and the people had soon perished together.

One thing John Locke leaves out of this story of political evolution as a response to war is an account of how the collective pursuit of war often caused an atrophy in respect for natural law justice between nations. A group of warriors raiding another tribe often encourage each other in theft, rape and militarily unnecessary murder that many of the individuals in that group of warriors would shrink from without such social support for depredations on the other tribe. If John Ferejohn and Frances McCall Rosenbluth are right that war fosters democracy, the price of democracy is high. 

For links to other John Locke posts, see these John Locke aggregator posts: 

• John Locke's State of Nature and State of War

• On the Achilles Heel of John Locke's Second Treatise: Slavery and Land Ownership

• John Locke Against Natural Hierarchy