While other considerations were obviously taken into account, notably financial, when Williams negotiated its driver's release to its rival earlier this year, the promise was a preponderant argument in Bottas' favour, according to Claire Williams.
"Valtteri has been an important part of Williams, he's got that fiery passion that you want in a driver because all he wants to do is be in the best car and win," deputy team principal Williams told Autosport.
"When you have a driver that you know that's all they want, why force them to race your car? That's not the right thing to do."
Over the summer of 2015, Ferrari debated whether to keep Kimi Raikkonen in the Scuderia's stable. Among its potential options was the hiring from Williams of Valtteri Bottas, but the Grove-based outfit said no.
"We stood in Valtteri's way once, when Ferrari came along, and I made a promise to Valtteri I wouldn't stand in his way again - I honour my promises.
"He has an amazing opportunity, and it will be interesting to see how Valtteri does this year."