Tokyo, Japan (CNN) -- Wearing a dark, Japanese style business suit, New Zealand environmental activist Peter Bethune sat in-between two uniformed court officers as his trial began on Thursday in downtown Tokyo.
The officers stared ahead, unflinching, as 44-year-old Bethune scanned the courtroom.
The head judge ran through each of the charges in Japanese. When the court interpreter began translating into English, Bethune turned to listen.
The charges are serious, announced the court: trespassing, damage to property, assault, forcible obstruction of official business and possession of an illegal knife.
If convicted on all counts, Bethune faces a maximum of 15 years behind bars.
Bethune was captain of Sea Shepherd's futuristic boat, the Ady Gil.
The batmobile-esque, $3 million boat collided with a Japanese whaling ship, the Shonan Maru 2, and sank in January.
Weeks later, Bethune jumped aboard the Shonan Maru 2 and attempted to make a citizen's arrest of the captain. He was arrested and brought back to Japan to face criminal charges.
In court Thursday, Bethune admitted to all the charges, except for assault.
"I admit that I boarded the Shonan Maru, but I believe that I have good reason to do so," he said. "I admit that I fired the butyric acid, but there were additional circumstances that we will discuss in court."
Prosecutors say the butyric acid burned two crew members of the Japanese whaling fleet, but Sea Shepherd calls it a harmless, albeit rancid, liquid. Butyric acid is found in rancid butter and vomit.
Bethune's Japanese attorneys have mounted a defense on the assault charge, which will be fought out until the trial ends on Monday.
Bethune's case is the first time a Sea Shepherd activist has been tried in a Japanese criminal court in the group's long-running battle with Japan's whalers in the icy waters of the Antarctic.
Japan annually hunts whales in the Antarctic, despite a worldwide moratorium on whaling, under the loophole that a country may legally do so if its purpose is scientific research.
Sea Shepherd, who notes the whale meat then gets sold in Japanese markets and served in restaurants, calls the science argument a sham.
Sea Shepherd's Seattle-based attorney, Dan Harris, is in Tokyo for Bethune's trial, though he is not Bethune's official Japanese counsel.
"This whole trial has been brought about for political reasons, far more than criminal reasons. If you look at what Peter Bethune did, he didn't do anything," Harris said. "He climbed aboard a Japanese ship. Nobody was in any danger. No one was under any threat. No one was afraid. For Japan to act like it's enforcing a criminal law is a little disingenuous, when you look at what Pete Bethune did."
Japan maintains that when its laws are broken, it will prosecute.
"We all recognize the right of protest, right of demonstration, right to express their views. That does not mean you can attack people with force, attack our vessels and crews with their vessels," said Joji Morishita, from Japan's Fisheries Agency.
Bethune testifies on Monday. A verdict is expected on June 10.