Existing　literature　on　system　defense　mainly　considers　single　or　multiple　intentional　impacts.　However,　intentional　impacts　(strategic　attacks)　and　unintentional　impacts　(natural　disasters)　can　happen　sequentially　in　practice.　To　overcome　this　limitation,　we　conduct　a　study　that　considers　the　optimal　resource　allocation　for　defending　a　k-out-of-n　system　against　sequential　intentional　and　unintentional　impacts,　which　is　motivated　by　the　defense　of　a　solar　power　grid　system　against　the　sequential　hacker＇s　attack　and　solar　storm.　We　study　the　optimal　defense　strategies　under　different　occurrence　orders　of　intentional　and　unintentional　impacts　and　analyze　the　impact　of　critical　parameters　on　the　defender＇s　optimal　defense　strategy　by　a　case　study.　We　also　investigate　the　scenario　in　which　the　defender　offers　supplemental　protection　to　the　elements　that　survive　the　first　impact.　The　case　study　indicates　that　deploying　as　many　elements　as　possible　in　the　first　stage　may　not　be　the　optimal　strategy　for　the　defender　no　matter　the　unintentional　impact　comes　first　or　the　intentional　impact　comes　first.　Providing　supplemental　protection　can　reduce　the　expected　loss　of　the　system　when　the　unintentional　impact　comes　first.