Boy and October are good, very good if you consider their age, but not not enough to compete directly with the best of the Jam, the Police or the Clash and that was always their goal. By the time of the release of War, the Jam had already disbanded, and the Clash were on the verge of collapse. The Police would disband before the release of TUF. So War is U2's manifesto. It is their proclamation: "we are the new kings".
As a manifesto it works, it is very eclectic, showing off a multitude of styles (similar to the Police's "Ghost in the Machine" or the Clash's "Sandistas" in that respect) There are multiple ideas on there within each song, which had it been produced by Enois, could've resulted in twice as many songs and twice as many discarded half-songs.
In a way War is the last true U2 album, very few US influences, no synths, no Eno adding textures, no Lanois; as such it is the culmination of 6years of work. They WANTED to experiment with violins, trumpets and backing singers and so they DID.
I also think that War is the reason that they could make TUF (and JT), not only because commercial (UK#1) and artistic success gave them some credit to persue other directions, but also because it is the perfect closer for a trilogy. After War (and UABRS) they could reset and start anew, much like after LoveTown or Popmart.
Lastly, War is their first "big" album, big in the sense that Boy and October deal with small, personal issues, where War tackles the big issues (conflicts on a global as well as interhuman scale). Boy and October are very much inward looking, Boy talks of a young man dreaming of taking on the world, War however, shows men actually taking on the world. Songs like Please, Crumbs or Love and Peace or Else, could not have existed without Seconds or SBS.