Complex sentence: Theodore was able to consolidate his eastern frontier for the remainder of the century, allowing the Nicaeans to later go over into the offensive in Europe, culminating in the reconquest of Constantinople in 1261.
Split into simpler sentences: Theodore was able to consolidate his eastern frontier for the remainder of the century. This victory allowed the Nicaeans to go over into the offensive in Europe as well, culminating in the reconquest of Constantinople in 1261.