Complex sentence: He had a dream (or "révülés"/meditation) of speaking with the head of Besermian army, so when he woke up, he knew they are preparing to attack, and he could do counteractions to protect the country.
Split into simpler sentences: He had a dream (or "révülés"/meditation) of speaking with the head of the Besermian army. When he woke up, he knew they are preparing to attack, and he could take action to protect the country.