But there can be little doubt that he really entered at once on a career of conquest, Lydia, at any rate, felt herself menaced by the new power, and seeing the danger which threatened the other monarchies of the time, if they allowed the great Arian kingdom to attack them severally with her full force, proposed a league whereby the common enemy might, she thought, be resisted with success.

Ambassadors seem to have been sent from Sardis to Babylon in the very year in which Nabonadius became king.

He therefore had at once to decide whether he would embrace the offer made him, and uniting with Lydia and Egypt in a league against Persia, make that power his enemy, or refuse the proffered alliance and trust to the gratitude of Cyrus for the future security of his kingdom.

It would be easy to imagine the arguments pro and contra which presented themselves to his mind at this conjuncture; but as they would be destitute of a historical foundation, it is perhaps best to state simply the decision at which he is known to have arrived.
This was an acceptance of the Lydian offer.

Nabonadius consented to join the proposed league; and a treaty was probably soon afterwards concluded between the three powers whereby they united in an alliance offensive and defensive against the Persians.
Knowing that he had provoked a powerful enemy by this bold act, and ignorant how soon he might be called upon to defend his kingdom, from the entire force of his foe, which might be suddenly hurled against him almost at any moment, Nabonadius seems to have turned his attention at once to providing means of defence.