Complex sentence: Nabopolassar was forced to endure Assyrian armies encamped in Babylonia over the next seven years, however he resisted, aided by the continuing civil war in Assyria itself which greatly hampered Sin-shar-ishkun's attempts to retake the parts of Babylonia held by Nabopolassar.
Split into simpler sentences: Nabopolassar was forced to endure Assyrian armies encamped in Babylonia over the next seven years. However, he resisted, aided by the continuing civil war in Assyria itself, which greatly hampered Sin-shar-ishkun's attempts to retake the parts of Babylonia held by Nabopolassar.