Gamillas is an aggressive empire, but it's not impossible to reach peace. More importantly, if the Yamato does not complete its lengthy journey as quickly as possible, they will be the only members of humanity left. They barely have time for this Pluto mission, which is only really being attempted for the sake of keeping Earth's morale from collapsing completely. If they attempt to bring down an empire spanning numerous star systems with a single vessel and somehow succeed, they will reduce mankind's population to under 1,000 people: the equivalent of choosing to engage in total nuclear war. While if they can successfully reach Iskandar, they can save millions of lives and keep the Wave Motion Gun as a deterrent to future invasions, the way that some third-world nations today attempt nuclear weapons programs.