If you read the Shincho-Ko ki often like I do, you will often see that Nobunaga isolated and starved his enemies into submission. It was a barbaric and effective tactic. David D. Neilson explains in his paper Society at War:
(p. 287) "Starvation sieges were one of Nobunaga's favored tactics and the grim scene that we encounter here is one that was played out many times in different locales. Was this sort of tactic cruel? Without question it was. But it was also a practical one that enabled an enemy to be reduced to the point of submission or utter defeat without wasting Nobunaga's most valuable resource, his soldier's lives, by sending them them against a hardened defensive fortification."
Neilson also does a great job explaining who was likely to be spared or killed.
(pp. 287-288) "Nobunaga's decisions regarding whether a garrison should be spared or put to the sword seem to have been primarily based on his personal perception of the enemy. If it was a former ally who had betrayed him like Araki or an enemy for whom he held great personal enmity like the Ikko Ikki, he was more likely to demand total extermination. A daimyo's vassals who were merely doing their jobs as loyal retainers were much more likely to be spared."
Tenka no tame!