A New Zealander goes on trial in Tokyo on Thursday on charges stemming from months of high-seas clashes between anti-whaling protesters and Japanese harpoon ships in the Southern Ocean.
Peter Bethune of the United States-based Sea Shepherd Conservation Society was detained more than three months ago and faces five charges in Tokyo District Court that could have him jailed for up to 15 years.
Mr Bethune, 45, will defend a charge of assaulting a whaler but will not fight four charges relating to his boarding of a Japanese ship, his American lawyer Dan Harris said.
The case will throw a spotlight on whaling, which Japan defends as part of its culture and carries out under a loophole to an international ban that allows lethal "scientific research", the AFP reports.
The Sea Shepherd group pursued and harassed Japanese whalers in Antarctic waters for months in the 2009-10 season, a campaign which both sides say reduced the Japanese cull by several hundred whales.
Mr Bethune was the captain of the kevlar powerboat the Ady Gil, which was sliced in two in a collision with the Japanese fleet's security ship the Shonan Maru II in January this year and sank soon after.
In February, Mr Bethune scaled the Japanese ship from a jetski with the intention of making a citizen's arrest of its captain for the attempted murder of the Ady Gil's six crew, and to bill him for the sunken vessel.
Instead, he was detained and taken to Japan, where he was charged with trespass, vandalism, obstructing commercial activities and carrying a knife when he boarded the ship - as well as assault for earlier allegedly injuring a whaler with a rancid butter projectile.
Japan, which says whaling has been part of its culture for centuries, has taken stern action against the activists, is also seeking Interpol's help to arrest Sea Shepherd founder Paul Watson for ordering his crew to harass whaling ships.
"What they are doing is nothing like peaceful protests but dangerous terrorist acts that threaten human lives, which should never be forgiven," says the Institute of Cetacean Research, which conducts Japan's whale hunts.