Since 2000 the US has spent about $6bn (£3.8bn) fighting drug production in Colombia and training its army to battle rebel groups.
Colombia is the world's leading producer of cocaine
The centrepiece of Plan Colombia has been the aerial spraying of coca plants, which yield the raw material for cocaine - which then helps finance the rebels.
Colombia is the world's top cocaine producer. Yet the US plan has proved highly controversial.
Policymakers argue it is in the national interest to fight cocaine at its source and to stabilise Colombia.
Critics agree - but say the current plan is ineffective, targets desperate farmers and has worsened an existing human rights crisis.
Under the plan, the US has given Alvaro Uribe's government more than $600m each year, heavily slanted towards military aid.
It has supplied helicopters, advisers, trainers and intelligence to the Colombian army to help it modernise and operate more effectively against both the coca farmers and rebels.
Congress initially specified that the aid should only be used against drug lords but the Bush administration has since indicated that some of it is being spent on counter-terrorism efforts.
When, in a November 2008 report, the US government's General Accounting Office (GAO) assessed progress between 2000 and 2006, there was some good news.
The US has financed training for Colombian troops
The Colombian armed forces had regained control of many areas formerly held by rebels and made inroads into their financing.
Murders and kidnappings had fallen as a result, the report said, and the number of fighters in rebel group Farc had fallen 50% to around 8,000.
But the goal of reducing the cultivation and distribution of illegal narcotics by 50% in six years had not been met.
Opium poppy cultivation and heroin production have fallen by half. But coca cultivation and cocaine production levels had increased by about 15% and 4% respectively.
Farmers had found ways to defy aerial and manual eradication programmes, by planting smaller patches or moving to new ground.
The report came at a tricky time. The US has indicated that aid to Colombia will have to be cut because of the current financial crisis.
A growing number of US lawmakers - including incoming President Barack Obama - have also voiced concern about human rights abuses in the country.
Days after Amnesty International accused the Colombian government of being "in denial" about the involvement of security personnel in extra-judicial killings, the head of Colombia's army resigned amid a scandal linking military officers to the murders of 11 civilians.
Local farmers have also complained about the aerial spraying, saying it kills all their crops, regardless of what they are, and affects their health.
The GAO report says that changes are needed.
It recommends that the US and Colombia "develop a joint plan for turning over operational and funding responsibilities for US-supported programs to Colombia" - although it does not give a time scale for this.
It also calls on USAID to assess whether its development assistance - about 20% of the total aid package - is helping to reduce the production of drugs in Colombia.
Some US Democrats are calling for more development - and less military - aid for Colombia.
With a new administration set to take office, the debate looks set to continue.