U.S. forces urgently need improvements in situational understanding to counter extremist enemies in highly complex environments. To address this challenge, the U.S. Army Research Laboratory (ARL) has established an active research program in Information Fusion. Information Fusion technologies provide human-machine capabilities for assessing and understanding complex situations by intelligently combining evidence from massive, multiple, heterogeneous information sources. ARL Information Fusion R&D is performed in partnership with industrial and academic partners, under ARL direction and management. The Brigade and Below Battlefield Awareness Network (B3AN) Program is a primary and promising ARL Information Fusion research program that seeks ways to improve defense situational understanding in tactical operations. B3AN currently includes four domain-specific web services: Real-Time Threat Prediction Fusion, Threat Over-watch in Urban Operations, Policy Aware Situational Understanding, and Real-Time News Analysis. This talk describes the defense situational understanding challenge, B3AN objectives, contributors, and research approaches. B3AN brings together investigators from the fields of service-oriented architecture, intelligent software agents, secure semantic web services, domain services frameworks, data fusion, predictive modeling, web and distributed data modeling, knowledge representation, and human-machine cognition.