File image - The South Africa contingent of the UN peacekeepers in Congo erect a razor wire barrier around Goma airport in the Democratic Republic of Congo.

The UN's peacekeeping mission in DR Congo is its largest and most expensive. But critics say the thousands of UN troops did nothing to stop M23 rebels from seizing the eastern city of Goma. Al Jazeera's Nazanine Moshiri looks at why the UN is staying on the sidelines in this conflict.