Complex sentence: The battles in Katanga by units of the 35th battalion marked the first time since the creation of the Irish State that an Irish Army unit went into battle against another nation's army.
Split into simpler sentences: The battles in Katanga by units of the 35th battalion marked a historical moment in Irish military history. It was the moment, when for the first time since the creation of the Irish State, that an Irish Army unit went into battle against another nation's army.