Given how Obito had acquired Bijuu in the past, why would he bother going to war?
Obito declared war because he knew that the alliance would keep Bee &amp; Naruto at a disclosed location since they had caught on to what the Akatsuki were doing. The war would allow him to draw out Naruto &amp; Bee to complete the plan of bringing back Ten Tails in it's prime state.

He had Kisame gather intel, but then of course Kabuto showed up. Obito's plan was essentially to gather intel on the locations of the last two jinchuuriki, use the zetsu to distract the allied shinobi forces in a war, and lure Naruto/Bee out. It didn't work out like that, but he had a legit plan.