Alliances and conflicts represent important features of complex systems like international relations. Such relations create a time-evolving signed network, where each node contributes in a unique manner to the global balance of the system. Therefore, a local index mathematically quantifying such a property becomes valuable. In this work, we introduce a local balance index for signed networks. We analyze its mathematical foundations and unique structural properties, differentiating it from existing local vertex invariants. We also establish a novel methodology linking changes in a nation's local balance to historical events. By scrutinizing the time series of local balance for countries between 1816 and 2014, we detect and categorize major historic events based on balance fluctuations. This approach harmonizes quantitative and qualitative analyses, and combined with the theory of "balance of power" is able to build up a new mixed approach to history based on network theory.
