Constans II took the throne when he was 11 so the early victories of his reign- the brief reconquest of Alexandria and several naval battles- can’t really be credited to him. But he was a quick learner and a talented general. At the age of 21 he personally led the eastern army into Armenia and drove the Arabs out. Five years later they were back and he repeated the feat in a quick campaign. By this time the Arab world was in the midst of a civil war and the worried Caliph bought a treaty with the promise of 1,000 gold pieces, a horse, and a slave for each day that Byzantium kept the peace.
Constans took the breathing room to reorganize his army and consolidate the weakened provinces. His most immediate concern was money. The imperial army had a serious morale problem. It showed a disturbing tendency to disintegrate in the east and revolt in the west- largely because its pay was so far in arrears. To fix the problem Constans had to control North Africa- where most of the surplus revenue came from- and that meant moving his center of operations closer to the threatened province. He transferred the government from Constantinople to Syracuse in Sicily and started to build up an army.
It wasn’t a popular move, but the emperor was determined. No less than three full-blown revolts broke out- one involving virtually the entire eastern army allied with the Caliphate and marching on Constantinople- but Constans refused to budge. Ultimately of course, this resulted in his embarrassing assassination. But if he had avoided that fatal bath would he have conquered the Duchy of Benevento? I don’t think so. His primary concern was with Africa. He attacked Benevento when he reached Italy for two reasons- to dissuade the Lombards from attacking him and to keep the surrounding Italians loyal. He had done much the same against the Slavs in the Balkans- successfully weakening them to neutralize a threat instead of outright conquest.
The man had a sensible plan and may even have had the skills to carry it out. (more about that here) Unfortunately, thanks to a soap dish, unfairly high taxes, and a disgruntled chamberlain, he never had the chance to carry it out.