本研究探討九一一事件後美國國土安全部（DHS）的成立、整合與治理轉型。研究旨在分析此一大規模制度改革為何未能完全解決其原先欲處理的協調失靈。本研究主張，關鍵問題在於正式整併與實質整合之間的落差。
本研究採取質性、理論導向的研究取徑，並採用單一個案研究設計，其分析主要依據美國政府官方文件、立法與監督資料、學術文獻及政策分析。本研究採用兩層次分析架構，在政治層次上結合危機治理與安全化理論，在制度層次上結合官僚政治與歷史制度主義。
首先，本研究發現，DHS 透過將二十多個聯邦實體納入單一內閣層級部門，並在新的制度架構下重組國土安全職能，達成了相當程度的正式整併。然而，其實質整合仍不完整，且仍受到限制。其次，九一一事件後的危機敘事、對威脅的安全化建構，以及將官僚碎片化界定為安全問題，為 DHS 的成立提供了政治動員與正當化的基礎。第三，官僚政治、資源競爭與制度慣性限制了 DHS 的長期治理轉型。本研究連結政治正當化與形塑治理結果的長期制度限制，並探討正式整併與實質整合之間的落差，從而有助於理解危機驅動的制度改革。

This study examines the establishment, integration, and governance transformation of the U.S. Department of Homeland Security (DHS) after 9/11. It addresses why this large-scale institutional reform did not fully resolve the coordination failures it was intended to address. The study argues that the central issue lies in the gap between formal consolidation and substantive integration.
This study adopts a qualitative, theory-driven approach and a single-case study design, drawing on official U.S. government documents, legislative and oversight materials, academic literature, and policy analyses. It employs a two-level analytical framework combining crisis governance and securitization at the political level with bureaucratic politics and historical institutionalism at the institutional level.
First, the study finds that DHS achieved a significant level of formal consolidation by bringing more than twenty federal entities under a single cabinet-level department and reorganizing homeland security functions within a new institutional framework. However, substantive integration remained partial and constrained. Second, the establishment of DHS was politically mobilized and legitimized through post-9/11 crisis narratives, securitized threat construction, and the framing of bureaucratic fragmentation as a security problem. Third, bureaucratic politics, resource competition, and institutional inertia constrained DHS’s long-term governance transformation. This study contributes to the analysis of crisis-driven institutional reform by linking political legitimation with the long-term institutional constraints that shape governance outcomes and by examining the gap between formal consolidation and substantive integration.

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