Complex sentence: The decision was landmark because up until that time, the legislature had freely regulated the powers of the governor and the decision created for the first time a legal basis for the governor to assume power in areas previously delegated to other officials.
Split into simpler sentences: The decision was noted because until then, the legislature had freely regulated the powers of the governor. The court created, for the first time, a legal basis for the governor to assume power in areas previously delegated to other officials.