Complex sentence: The next year (b.c.217) he concerted a plan with Lycurgus king of Sparta for the invasion of Messenia, but failed in the execution of his part of the scheme, being repulsed by the Cyparissians before he could effect a junction with Lycurgus.
Split into simpler sentences: The next year (217 BC) he concerted a plan with Lycurgus, king of Sparta, for the invasion of Messenia. However, he failed in the execution of his part of the scheme, being repulsed by the Cyparissians before he could effect a junction with Lycurgus.