They had long ago made up their minds to act thus, but they were looking for a suitable opportunity and a pretext that would appeal to foreign nations.
For the Romans very rightly paid great attention to this matter, since, as Demetrius says, when the inception of a war seems just, it makes victory greater and ill-success less perilous, while if it is thought to be dishonourable and wrong it has the opposite effect.
So on this occasion their disputes with each other about the effect on foreign opinion very nearly made them desist from going to war.
Walbank Commentary