Most　existing　research　on　system　defence　is　restricted　to　passive　measures　such　as　providing　redundancy,　protecting　system　elements,　and　deploying　false　elements.　Some　recent　works　have　considered　the　launching　of　preventive　strikes,　where　a　preventive　strike,　if　successful,　destroys　all　the　attacker＇s　resources.　However,　in　practice,　a　preventive　strike　might　cause　a　random　loss　to　the　attacker＇s　resources.　This　paper　assumes　that　the　destroyable　attacking　resources　follow　a　Poisson　distribution.　Procedures　are　presented　to　solve　the　optimal　defence　strategies　for　three　cases:　(1)　where　preventive　strikes　and　object　protections　are　the　only　available　defence　measures;　(2)　where　false　targets　can　also　be　deployed,　and　they　are　perfect;　and　(3)　where　false　targets　can　also　be　deployed,　but　they　are　imperfect.　Numerical　examples　are　provided　to　illustrate　the　applications,　and　sensitivity　analysis　is　employed　to　demonstrate　the　influence　of　parameters.