[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] This dissertation addresses two issues in the philosophy of war: the prevention of unjust wars, and the moral justification of lethal defense against 'lesser aggression' (a kind of international aggression that involves the threat to use force rather than the actual use of force). Regarding the prevention of unjust wars, I draw on the bargaining perspective on war, from political science. According to this perspective, war between disputing states can occur when those states are unable to bargain their way to a peaceful resolution of their dispute. The perspective thus suggests that war can be avoided by preventing bargaining failure, and I argue that the wars that would be avoided by preventing bargaining failure will tend to be unjust wars. Therefore, preventing bargaining failure will tend to prevent unjust wars. I then propose some specific moral duties of states to prevent bargaining failure (in order to prevent unjust wars). Regarding the moral justification of lethal defense against lesser aggression, I argue for two things. First, I argue against David Rodin's view that considerations of deterrence do not help to justify lethal defense by a victim against a lesser aggressor. I argue, contra Rodin, that there is good reason to believe that a victim's lethally resisting a lesser aggressor will sometimes deter future aggression against itself and that this 'deterrence effect' may be a significant moral benefit of such resistance (thus helping to justify the resistance). Second, I argue that, under certain empirical conditions, a victim's lethally defending against a lesser aggressor is needlessly morally costly, hence morally unjustified. This is because a victim of lesser aggression will often have available an alternative defensive option--which I call returning-the-threat--that is significantly less morally costly than lethal resistance.