The exchange last year between the U.S. Embassy in Tokyo and the State Department shows the two countries were negotiating before an International Whaling Commission meeting, The Guardian reports. The cables were among the thousands of documents obtained by WikiLeaks.
Under the U.S. proposal, Japan would have cut its annual quota in the Antarctic. In exchange, Japanese whalers would have been given the right to hunt other species in their own waters while the United States would have adopted legislation to "guarantee the security of the seas" by cracking down on the anti-whaling group Sea Shepherd.
Sea Shepherd, which operates ships flying the skull-and-crossbones pirate flag, follows the Japanese whaling fleet every year, attempting to disrupt its operations.
A majority of IWC nations last year rejected the compromise with Japan.
Paul Watson, Sea Shepherd's founder, said Wednesday if the United States did investigate the group it found no laws were broken.
"For Sea Shepherd, the most important part of this document is the declaration by Japan that Sea Shepherd has been responsible for the whaling fleet not reaching their quotas for the last few years," he said. "This completely validates Sea Shepherd's actions as effective."