Over the past decades, migration has been used as a political weapon. In order to better understand this security issue, academic attention is needed to develop coherent policy tools. Among academia, there is a debate on the vulnerability of states to coercive migration diplomacy. Greenhill’s (2010) theory of normative constraint advances eight independent risk factors that make liberal democracies more vulnerable to coercion. However, this theory ought to be supported by qualitative research to understand how these variables are linked to coercive migration diplomacy. A theory testing process tracing (TTPT) single case study is followed to accomplish such aim. The key research question in this thesis is: how do liberal democracies succumb to coercive migration diplomacy? It therefore focuses on refining normative constraint theory, as well as on the effect of border control. The migration crisis between the EU and Türkiye first in 2015-2016 and then in 2019-2020 is used as a case study. The results of this thesis suggest that factors including migrant camp mobilization, codified commitments to refugee protection, and pluralistic politics can interact in a sufficient way to make liberal democracies vulnerable to coercion. Furthermore, this thesis has found that an effective border control is an additional factor to consider when assessing the vulnerability of a target state to concede to the demands of a coercer. This made the first instance of migrant instrumentalization a success, albeit enhanced border control capacities on the Greek side in conjunction with other factors made Türkiye’s second coercive attempt fail. Since the findings of this thesis are qualitative in nature, they can only be directly applicable to this case. However, policy lessons can be drawn from this study.