Complex sentence: Guaimar promptly offered them numerous incentives to stay, but to no avail, the Normans returned to France, promising to spread the word about the need for fighting men in the south.
Split into simpler sentences: Guaimar promptly offered the Normans numerous incentives to stay, but to no avail. Before they left, however, the Normans promised to spread the word about the need for fighting men in the south.