Regarding the Jan. 15 editorial “Mission shrink in Afghanistan”:

The Post is correct to point out that the number of troops President Obama is willing to leave in Afghanistan will reflect the U.S. commitment to the relationship between the two countries. This is largely an exercise of doing the math: Take the number of American soldiers necessary to carry out a counterterrorism mission, then multiply the number of Americans needed to advise, train and enable by the number of Afghan units that need our support. Add those numbers together to come up with a total.

Two months ago, Kimberly and Frederick Kagan did this exercise for The Post and arrived at more than 30,000. Former U.S. ambassador to Afghanistan Ronald Neumann, also writing in The Post, warned of making the numbers too small and called any policy that tried to pursue purely a counterterrorism strategy “bankrupt.”

Mr. Neumann is certainly right, and one also wonders why President Hamid Karzai would accept U.S. Special Operations troops operating in Afghanistan without the quid pro quo of advisory and technical assistance to his national army. Doing the math will get you a big number. Going with a small number will not be credible — and will be dangerous to those remaining behind.

Alfred R. Barr, Washington