What was the justification for attacking or fighting the Persians in 'Iraq by the Caliphs Abu bakr and 'Omar?

I mean fighting for the ridda wars at least seems obvious. People have refused to pay Zakat and became against the Muslim rulers by for example following the false prophets like Musailimah and Sajjah. But what is the correct justification (or historical background) for attacking 'Iraq? If these were preventive strikes, so how did the Muslims come to know about any attack or conspiracy? If not what was the motivation? I wonder if jizya played a role... Especially when one reads that many sahaba have became rich.