ISLAMABAD (AP) — The U.S. and NATO have promised to pay $4 billion a year until 2024 to finance Afghanistan's military and security forces, which are struggling to contain an advancing Taliban. Already, the U.S. has spent nearly $89 billion over the past 20 years to build, equip and train Afghan forces.

Yet America's own government watchdog says oversight of the money has been poor, hundreds of millions of dollars have been misspent and corruption is rife in the security apparatus.

Monitoring where the future funding goes will become virtually impossible after Aug. 31, when the last coalition...