Should scientists have morals?
This question undoubtedly formed a substantial concern both for Leonardo da Vinci and for Vannevar Bush, although both of them seem to have resolved their dilemma with the same conclusion. They both regarded the application of their scientific knowledge and skills to military matters as permissible insofar as their efforts would aid mostly defensive measures and / or their work would remain purely on a theoretical level, with no active responsibility being taken for any harm done. Furthermore, the justification of such research would be slightly easier for both scientists since military funding has always been a highly satisfactory venue of financial income throughout history. In the end, however, both could have legitimate reasons for choosing to contribute to the development of military technologies: Any invention which may have applications in warfare could potentially have other uses during times of peace. Furthermore, technology could bring about more humane ways of achieving destruction. This paradoxical statement will be explained shortly.
Leonardo da Vinci, for instance, designed mobile ladders with which to storm fortresses, although, much in the style of Vizzini from the Princess Bride, he quickly supported the opposing side of the conflict by then designing walls which would be able to defend against an attack by his own ladders.
Leonardo further designed the first tank which would not see the light of day until some four hundred and fifty years later. His particular model would require eight soldiers to operate, and would be utilized mainly to penetrate the enemy fronts without any injury to the operators of the 'covered vehicle'. The infantry would follow the tank, suffering minimal losses while travelling far into the enemy lines. Leonardo likened his tank to a mechanical version of an elephant. His cannon was perhaps his most violent design, with eleven nozzles which could fire simultaneously. The shots could cover a wide region, with the explosion of the central cannonball setting fire to the two cannonballs nearest to it, and so on recursively. All the balls would catch fire "in the time it takes to say a Hail Mary."
We will never know if Leonardo meant to repent for having designed such a deadly weapon, or if he simply happened to be on one of his more mischevious days when he penned that particular page.
Vannevar Bush also contributed extensively to military research during his time. One of his first projects was contributing to the development of submarine detection systems during World War I, a mainly defensive measure. The use of his differential analyzer for military purposes during World War II similarly constitutes a somewhat passive application. However, his involvement with the Manhattan Project raises justifiable concerns. How can a scientist, whose main motivation should be that of trying to improve the condition of human life, work towards destroying it?
This example constitutes what was referred to as a 'more humane way of achieving destruction' above. Most scientists involved with the Manhattan Project wished never to see their work in action. To them, it seemed that the danger that the atom bomb posed to other countries should be more than reason enough to seek alternatives to military conflict. Clearly, the most humane way of achieving destruction would be seek an alternative to it, and Bush may have been motivated by this line of thought. He was clearly convinced that science should be utilized for the improvement of human civilization at all costs, as he asserted in his post-WWII report to Roosevelt. Bush noted that using the technologies improved during the war in providing access to and control over the sum of all inherited knowledge of human civilization should form the foremost motivation of scientists, clearly making a veiled reference to his Memex.
In short, both Leonardo da Vinci and Vannevar Bush contributed to military technologies, which might be reason enough to blame them. Had they chosen not to do so, soldiers all over the world today might be using swords, bows and arrows, axes and possibly handguns in battles that erupted almost continuously since there seemed to be little reason for nations to refrain from war.
It's hard to say which quantum branch would be more favorable. We can only guess that both scientists did what they judged to be right, and hope that the judgment of such bright minds will provide a better world for humanity.