Sofar,causesoftheKorean Warhavebeenmainlyinvestigatedboth in the internationalpolitics and domestic politics. On the otherhand,the aspects ofmilitary strategies been researched. As CarlVon Clausewitz said,‘Warisa politicalmeans',causesofwarshould bediscussed with politicalmotives,butmilitarystrategicaspectsareveryimportantaswell. Itisdifficultforacountrytostartawarwithoutexpecting‘possibilityof victory',nomatterhow attractiveapoliticalmotiveseems. Possibility of victory can be calculated through both sides'military strategies.Therefore,thisstudyaimstoinvestigatethereasonswhySouth KoreafailedtodeterNorthKorea'sinvasionfrom theperspectiveofmilitary strategy. Thus, this study examines both the attacker’s and defender's military strategicadvantagesand disadvantagesaswellasthecorrelation between militarystrategiesandwardeterrence.Basedon thefindings,this study investigatesthetwoKoreas'military buildup and military strategies before the Korean War,furtheranalyzing the reasons why South Korea failedtorepelNorthKorea'sinvasionfrom thepointofmilitarystrategy. When it comes to North Korea's military buildup and invasion plan between 1948 and 1950, North Korea had clear political and military objectives called ‘Unification underCommunism'and adopted a blitzkrieg strategy to attain its objectives in the shortestperiod,and its military buildupwasgoodenoughtoexecuteitsblitzkriegstrategy. When it comes to South Korea's military buildup and defense plan,however,South Korea failed to establish an exactdirection between its politicaland militaryobjectives,which could bethedefenseagainstNorth Korea'sinvasion and restoration ofNorth Korean territoryand even failed toestablish adefenseplanbasedon anattrition strategywhich waslikely to cope with North Korea's blitzkrieg strategy mosteffectively.Moreover,South Korea didn't properly pursue military buildup because ofseveral restrictions. In conclusion,North Korea's possibility ofvictory became considerably higherfrom thepointofmilitarystrategyin theearly1950s,which ledto acircumstancethatSouthKoreacouldn'tdeterNorthKorea'sinvasionfrom amilitarystrategicpointofview aswellasfrom theinternationalpolitical pointofview.