Complex sentence: The Axis may declare war at any time, while the US may declare war at the collect income phase of their 3rd turn, the USSR at the combat move phase of their 4th turn.
Split into simpler sentences: The Axis may declare war at any time, while the US may declare war at the collect income phase of their 3rd turn. On the other hand, the USSR may declare war at the combat move phase of their 4th turn.