In this article, we focus on the scholarly and policy debate on autonomous weapon systems and particularly on the objections to the use of these weapons which rest on jus ad bellum principles of proportionality and last resort. Both objections rest on the idea that AWS may increase the incidence of war by reducing the costs for going to war or by providing a propagandistic value. We argue that whilst these objections offer pressing concerns in their own right, they suffer (...) from important limitations: they overlook the difficulties of calculating ad bellum proportionality; confuse the concept of proportionality of effects with the precision of weapon systems; disregard the ever-changing nature of war and of its ethical implications; mistake the moral obligation imposed by the principle of last resort with the impact that AWS may have on political decision to resort to war. Our analysis does not entail that AWS are acceptable or justifiable, but it shows that ad bellum principles are not the best set of ethical principles for tackling the ethical problems raised by AWS; and that developing adequate understanding of the transformations that the use of AWS poses to the nature of war itself is a necessary, preliminary requirement to any ethical analysis of the use of these weapons. (shrink)