In context, it's clear that counterattacking with excessive force is something that one should not do. “Second eleven” is an adjectival phrase that qualifies something as inferior in some way.
In what way, precisely, is “second eleven” inferior? A web search suggests that this is about second-rate sports teams, though I don't fully understand the nuances of what it means for a cricket team to be “second XI”. But does “second eleven” necessarily mean weaker? In the context of the novel, either “gauche” or “having the potential to trigger a war” seems more plausible.
In addition to understanding the exact nuance of the meaning, I'm curious where and when this expression would be used outside of a sport context.