Japan started the war with a surprise attack, but Russia intended to go to war with Japan and everyone knew it. Japan just figured that, as the smaller country, it ought to try to seize what advantages it could
Hmmm. Politics of the 1800s are weird. Both sides were imperialists, expanding their domains far beyond their historical norms.
Russia expanded eastward. Japan expanded westward.
You're right that Japan ultimately was the one who started the attack though. But I'm also looking at Russia's expansion to seek a warm-water port on the Pacific Ocean and connect it up with the Tran-Siberian railway.
Russia definitely was encroaching upon the East. But Japan also was encroaching upon the West, with the 1890s Sino-Japanese War. Soooooo... both are kind of imperialistic assholes on this event, now that I think of it.