Israel's 12-day war against Iran and the US's direct entry into this conflict rank among the most important geopolitical and security developments in West Asia in 2025, affecting the regional balance of power and prevailing security order beyond a military confrontation. The research examines what factors and strategic considerations led to the US's direct entry into Israel's 12-day war against Iran, and what results and consequences this intervention brought for the regional balance of power. Conducted via a descriptive-analytical method using library resources, official documents, think-tank reports, and reliable scientific sources, the data were analyzed within the balance of threat theory. Findings show the direct US intervention resulted from growing threat perception due to Iran's enhanced deterrence, missile and drone capabilities, expanded regional influence, the need to maintain Israel's strategic superiority, preserve US deterrence credibility, prevent shifts in the regional balance of power, and sustain Washington's preferred security order. This intervention strengthened the US military presence in West Asia, intensified geopolitical competition, altered the deterrence model, increased security instability, and redefined the regional balance of power. Results indicate the balance of threat theory better explains the causes and consequences of this US strategic behavior than other realist approaches.