OKUBO didn't think Daewongun lend an ear to persuasion, and thought SAIGO will be killed (even if he won't be killed, if Daewongun rejected the envoy, it can be a legitimate reason for outbreak of war) inevitably when he went to Korea, and he felt a sense of danger that they will start the war with Korea as a result, and his strategic decision that Japan doesn't have national strength at that time to strain the relationship (as a result, wage a war) with Korea, Qing, and eventually Russia, an emotional backlash to the remaining group that ignored agreement and about to dabble in diplomatic gambling, he strongly opposed because of the presence of a diplomatic matters (Ryukyu attribution issue with Qing [see the Taiwan expedition], territorial right problem on Sakhalin and Kurile Islands with Russia, territorial right problem on Ogasawara Islands [Bonin Islands] with England, revision of the unequal treaty) that must be dealt with before Korean Peninsula problem on Japanese international standing, and he correlated it with expenses etc and appealed postponement by telling disadvantage of subjugation of Korea.
大久保は、説得に大院君が耳を貸すとは思えず西郷が朝鮮に行った場合必ず殺される（殺されずとも大院君が使節を拒否した場合は開戦の大義名分になってしまう）、そうなった場合結果的に朝鮮と開戦してしまうのではないかという危機感、当時の日本には朝鮮や清、ひいてはロシアとの関係が険悪になる（その帰結として戦争を遂行する）だけの国力が備わっていないという戦略的判断、外遊組との約束を無視し、危険な外交的博打に手を染めようとしている残留組に対する感情的反発、朝鮮半島問題よりも先に片付けるべき外交案件が存在するという日本の国際的立場（清との琉球帰属問題（台湾出兵参照）、ロシアとの樺太、千島列島の領有権問題、イギリスとの小笠原諸島領有権問題、不平等条約改正）などから猛烈に反対、費用の問題なども絡めて征韓の不利を説き延期を訴えた。