With the growing tensions in the Pacific, Kashii was subsequently (31 July 1941) assigned to the Southern Expeditionary Fleet under Vice Admiral Jisaburo Ozawa.

On 5 December 1941, Kashii departed Cap St. Jacques, French Indochina escorting seven troops of transports carrying the Imperial Japanese Army's 143rd Infantry Regiment to Kra Isthmus (Thailand) and Malaya, and was thus still en route at the time of the attack on Pearl Harbor.

Kashii continued with patrol duties in the eastern Indian Ocean off Burma, the Andaman Islands, and Penang through September.

On 21 September 1942, Kashii departed Saigon on an emergency transport mission to reinforce Japanese forces in the Solomon Islands.

In January 1943, Kashii underwent retrofit at Keppel dockyard in Singapore, to truncate its masts and add a "submarine spotting station" to its foretop.

[citation needed] From 24 July 1943 – 22 August 1943, Kashii made two transport runs carrying troops and supplies to Port Blair and Car Nicobar.

On 29 August 1943, off Pulo Weh, north Sumatra en route to Sabang, Kashii was attacked by the Royal Navy submarine HMS Trident, which fired all eight of its bow torpedoes but missed.

On 25 March 1944, Kashii was reassigned directly to Headquarters, General Escort Command, and was modified for anti-submarine warfare at Kure Naval Arsenal.

The convoy arrived safely in Manila, and after offloading aircraft, proceeded to Singapore, returning to Moji without incident on 15 August 1944.

The convoy was attacked on 16 September 1944 by the submarines USS Queenfish and Barb, which sank two oilers and the aircraft carrier Un'yō.

On 10 December 1944, Kashii was reassigned to the 1st Surface Escort Group and departed Moji for Takao, Taiwan with a convoy of Army transports.

Kashii was hit starboard amidships by a torpedo from a Grumman TBF Avenger, then a Curtiss SB2C Helldiver struck with two bombs aft, setting off the depth charge magazine.