This Bachelor's thesis examines the consequences of Sweden's NATO membership regarding its security and defence policy, with a particular focus on Gotland. Sweden's transition to NATO membership was motivated by escalating geopolitical tensions in Europe due to the conflict between Russia and Ukraine and especially in the Baltic Sea region. This study employs a qualitative approach, using semi-structured interviews with key stakeholders in Gotland's military and municipal sectors to assess the strategic, operational and tactical effects of NATO membership. The results indicate that although NATO membership strengthens Sweden's national defence and geopolitical position, its immediate tactical and operational effects on Gotland are limited. The ongoing military development in Gotland, which was initiated before NATO membership, continues to develop with additional support and cooperation opportunities provided by the alliance. The research highlights the strategic necessity of NATO membership for Sweden, which provides increased security guarantees and adapts to broader geopolitical goals. However, the full extent of NATO's influence on Gotland's military development and societal impact requires further investigation given the recency of Sweden's membership on March 7, 2024. This thesis contributes to the academic discussion of geopolitical alliances and provides insights for policy makers in strategic planning and implementation.