Final answer:
The most likely reason for two tribal groups to integrate temporarily into a single unit is mutual defense against a common threat, as this has been a historical catalyst for such alliances, trumping other factors like shared economic interests or cultural exchange.
Step-by-step explanation:
The most probable reason for two separate tribal groups to integrate temporarily into a single unit is mutual defense against a common threat. This integration, often under the leadership of a capable or charismatic individual, is typically a response to external pressures that require collective action for survival. Throughout history, tribes have united to form confederations or larger tribal units to face challenges such as military threats from neighboring societies, the need for effective resource management, or to engage in collective actions like raids or trade. Similarly, while economic interests or political consolidation could also be factors, the need for defense against external threats has often been a primary driver for temporary tribal integration.
Comparatively, reasons like cultural exchange and intermarriage or shared cultural identity tend to foster longer-term alliances, and while important, they are usually not the immediate cause for temporary unification. Instead, the urgency of defense often precipitates such alliances, with other benefits such as cultural exchange or economic cooperation emerging as a result of the newfound unity.
Looking at the broader spectrum of human organization, the rise of states and empires also underscores the role of integrative and conflict pressures in political consolidation. As such, the need for defense and mutual support in times of conflict emerges as a strong catalyst for temporary alliances across different scales of human society, from tribal groups to nation-states.