Yonaguni - Japan began its first military expansion at the western end of its island chain in more than 40 years yesterday , breaking ground for a radar station on a tropical island off Taiwan.
The move risks angering China, locked in a dispute with Japan over nearby islands which they both claim.
Japanese Defence Minister Itsunori Onodera, who attended a ceremony on Yonaguni island to mark the start of construction, suggested the military presence could be enlarged to other islands in the seas south-west of Japan's main islands.
"This is the first deployment since the US returned Okinawa (in 1972) and calls for us to be more on guard are growing," Mr Onodera told reporters. "I want to build an operation able to properly defend islands that are part of Japan's territory."
The military radar station on Yonaguni, part of a longstanding plan to improve defence and surveillance, gives Japan a lookout just 150km from the Japanese-held islands also claimed by China.
Building the base could extend Japanese monitoring to the Chinese mainland and track Chinese ships and aircraft circling the disputed isles, called the Senkaku by Japan and the Diaoyu by China.
The 30 sq km Yonaguni is home to 1,500 people and known for strong rice liquor, cattle, sugar cane and scuba diving. Prime Minister Shinzo Abe's decision to put troops there shows Japan's concerns about the vulnerability of its thousands of islands and the perceived threat from China.
Beijing's decision last year to establish an air defence identification zone in the East China Sea, including the skies above the disputed Senkaku/Diaoyu islets, has rattled Tokyo.
Tapping concern about China, Mr Abe raised military spending during the last fiscal year for the first time in 11 years to help bolster Japan's capability to fight for the islands with a new marine unit, more longer-range aircraft, amphibious assault vehicles and helicopter carriers.
Mr Onodera's groundbreaking ceremony on Yonaguni took place four days before President Barack Obama lands in Tokyo for a summit with Mr Abe, the first state visit by a US president in 18 years.
The United States, which under its security pact with Tokyo has pledged to defend Japanese territory, has warned China about taking any action over the disputed islets, but has not formally recognised Japan's claim of sovereignty over the territory.