General Douglas MacArthur accelerated the mobilization efforts to bring the Philippine Army numbers to 120,000, in ten divisions.

By September 1941, the senior officers undertook the Command and General Staff Course (CGSC) in Baguio, which was concluded on November 17, 1941.

The area of mobilization for the 21st Division covered the provinces of Tarlac, Pangasinan, La Union, and Nueva Ecija.

On December 20, Capinpin and all division commanders under the USAFFE with a rank of colonel were promoted to brigadier general.

[5] World War II broke out when Imperial Japanese Army bombers raided Baguio, Fort Stotsenburg, and Iba on December 8.

While it was predicted that the bulk of the Japanese invading force would land in Lingayen Gulf, the 14th Area Army focused on the eastern shores at Agoo, La Union.

[10] Gen. Capinpin finally became the Adjutant General of the fledging Armed Forces of the Philippines after World War 2, and retired from military service on April 30, 1948.