If we can keep pouring in weaponry, at some point Ukraine will start more counteroffensives. As the assessment notes, the Russians are already taking up defensive positions. Eventually, I’m sure that the Ukrainians would want to not just break the siege at Mariupol, but cut the Russian lines in half and encircle them to the west, to provide a buffer for Odessa.
How did Ukraine manage to attack a key port and sink a ship well behind Russian lines? No one’s quite sure yet, the Wall Street Journal reports, but the apparent success may create even more problems for Russia’s attempts to seize Mariupol. Berdyansk had been a key supply point, and seemingly out of range of Ukraine until this morning.
Expect to see them take more initiatives like this.
Oh, and by the way, I don’t think this would be “World War III.”
What we now call WW I wasn't really a world war; it largely involved Europe, and didn't involve Asia at all. Similarly, if NATO goes to war with Russia, it will primarily be a European war, but unless China gets involved (unlikely) it won't be a world war.
The only way that this war will become a world war is if China decides to simultaneously (or not long afterward) take Taiwan, which would drag Asia (and maybe America) into the conflict.