What significant action did NATO take in 2003?

The action that NATO took in 2003 involved assisting the United Nations with security in Afghanistan. This occurred after the establishment of the International Security Assistance Force (ISAF) in late 2001, which NATO took command of in 2003. The intention was to help maintain security in Afghanistan following the overthrow of the Taliban regime and to support the Afghan government in rebuilding the country. This marked a significant step for NATO, as it demonstrated the alliance's commitment to collective security beyond its traditional boundaries in Europe and into crisis management and peace support operations.

The other options reflect actions that either were not taken by NATO or were related to different contexts. For instance, while NATO has conducted peacekeeping missions in Europe, this was not a significant action in 2003. Moreover, NATO did not provide humanitarian aid in Syria, nor did it engage in military operations in Iraq during that year, as the predominant international forces in Iraq at the time were led by the United States and the coalition of other nations, not NATO itself.