When it comes to Morocco’s illegal annexation of Western Sahara, three theories stand out to explain several decades of Moroccan expansionism: Diversionary foreign policy (after two failed military coups), geopolitical realism (to create a buffer zone vis-à-vis the regional rival Algeria) and resource-based colonialism (the main argument raised by Sahrawi nationalists). In contrast, this paper seeks to analyze Morocco’s rule over Western Sahara as a case of postcolonial state expansion. The phenomenon of postcolonial state expansion can be defined as the systematic and long-term expansion of postcolonial state institutions across contested borders and the resulting coercive rule over a neighboring territory and its population. Comparable examples might include the Jordanian annexation of the West Bank, China’s rule over Tibet and the Indonesian occupation of East Timor. Based on insights from historical institutionalism, Morocco’s territorial expansion is thus explained as a long-term effect of colonial rule in the Maghreb. While the drive for Muhammad Allal al-Fassi’s ‘Greater Morocco’ is often depicted as an anachronistic case of imperial ambition, this aspect of Moroccan nationalism actually falls into a broader category of postcolonial irredentism typical for nation-states emerging from the artificial territorial divisions of colonial rule. Yet while Morocco’s slow-moving process of territorial recovery at first went unchallenged both in the North (the Spanish Protectorate in 1956) and in the South (Cape Juby in 1958, Ifni in 1969), Morocco’s claim to Western Sahara in 1975 clashed with another legacy of colonial rule, namely Sahrawi nationalism based on the internationally recognized Sahrawi right to self-determination. In order to contribute to comparative research on territorial expansionism by postcolonial states, the paper discusses both Moroccan policies of political integration as well as different Sahrawi strategies of resistance. The unique aspect about postcolonial state expansion is analyzed as a highly paradoxical opportunity structure for newly independent states emerging from European empire: While postcolonial irredentism and Cold War alliance politics made territorial expansion both domestically attractive and geopolitically feasible at first sight, in the long run these cases of state expansion conflicted with the fundamental normative shift towards self-determination that came to define the postcolonial era.