Complex sentence: The army's leaders realised that they had overestimated their ability to pursue operations against heavily defended urban targets, such as the strategic central Croatian town of Gospić, which the JNA assessed as potentially a "second Vukovar".
Split into simpler sentences: The army's leaders realised that they had overestimated their ability to pursue operations against heavily defended urban targets. The central Croatian town of Gospić, for instance, was a strategic target of the JNA but was now assessed as potentially a "second Vukovar".