Answer:

Introduction:

Multilateralism, which refers to the practice of coordinating national policies in groups of three or more states, has been a cornerstone of international diplomacy, particularly after World War II. However, the post-Cold War era, characterized by the end of bipolar power structures, has witnessed a noticeable decline in multilateral cooperation. A myriad of reasons has contributed to this weakening, both at the global and regional levels.

Body:

Factors contributing to the decline of multilateralism at the global level:

• Rise of Unilateralism: Major powers, particularly the U.S. under the Trump administration, took unilateral decisions such as withdrawing from the Paris Agreement on climate change and the Iran Nuclear Deal, sidelining multilateral mechanisms.
• Emergence of New Powers: The rise of countries like China and India challenges the post-WWII order, which was largely dominated by the West. This led to the creation of new forums like BRICS, diluting the dominance of traditional multilateral institutions.
• Shift towards Bilateral Trade Agreements: Many countries are favoring bilateral trade agreements over multilateral ones. For example, the U.S.-China trade war, where two major powers opted for bilateral negotiations rather than using WTO mechanisms.
• Distrust in Global Institutions: There’s a growing perception that institutions like the UN, WTO, and WHO are inefficient or biased. For instance, the recent criticisms of WHO’s handling of the COVID-19 pandemic. 

Factors contributing to the decline of multilateralism at the regional level:

• Regional Power Struggles: In regions like the Middle East, power struggles between nations like Iran and Saudi Arabia make multilateral solutions difficult.
• Rise of Nationalism: Countries are increasingly prioritizing their national interests. For instance, Brexit, where the UK chose to leave the EU, a beacon of regional multilateralism.
• Mismatch of Regional Goals: In regions like Southeast Asia, the diverse economic and political goals of countries make it hard to forge multilateral agreements. The recent RCEP (Regional Comprehensive Economic Partnership) did materialize, but India opted out, citing disparities in regional interests.
• Security Concerns: In regions like South Asia, longstanding issues like the Kashmir dispute between India and Pakistan overshadow potential multilateral cooperation.

Events like global health crises, conflicts, and economic downturns further strain multilateral mechanisms. New alliances may emerge, but they may not necessarily follow the multilateral ethos of the post-WWII era.

Conclusion: 

The decline of multilateralism in the post-Cold War era can be attributed to a mix of global power dynamics, regional tensions, the resurgence of nationalism, and a declining trust in international institutions. While the world stands at a crossroads, there’s an essential need to revive and restructure multilateralism. This is vital not just for peace and prosperity, but to address global challenges that single nations cannot tackle alone, like climate change and pandemics.