With more and more success of Software Defined Networking (SDN) in academia and industry, people have started to explore how to integrate SDN into military coalitions, to realize a software-defined coalition (SDC) infrastructure. However, the integration of SDN into SDC is non-trivial due to the insufficiencies of the SDN datapath for expressing the data plane behavior in SDC systems. In particular, SDC systems operate in highly dynamic tactical networks (e.g., wireless networks) and requires the data plane to support a wider range of events other than the traditional incoming packet event in wired SDN. In addition, SDC systems widely adopt in-network processing (INP) such as network coding, and supporting these functionalities in the data plane requires flexible storage for packets and complex operations on both packet headers and payload, which are not supported in the existing SDN datapath. In this paper, we tackle these issues by designing OpenSDC, a novel, generic SDC datapath which extends the current match-action primitive, with three new primitives: event, packet buffer, and packet INP block. The implementation of the proposed primitives can be optimized using a range of techniques to accelerate the event processing efficiency of OpenSDC. We implement a prototype of the proposed datapath, and demonstrate its ability to support network-coding-based 1+1 data delivery protection in dynamic tactical networks. Evaluation results show that compared with a state-of-the-art proactive protection system implemented using only match-action tables, our prototype significantly improves the efficiency and resiliency of tactical networks.