Britain's Defense Ministry said the last flight carrying UK army personnel has left Kabul, ending the evacuation mission.
"We should be proud of our armed forces, welcoming to those coming for a better life, and… sad for those left behind," Wallace said on Twitter.
"Our obligation to them does not end with our leaving," he added.
With the UK's 20-year mission to Afghanistan coming to an end, Wallace said that there would be "many lessons to learn," but there were also "endless examples of amazing achievements."