The session is intended to help pave the way for key decisions to be announced at a summit meeting of NATO heads of government in Chicago in May.
But France this week appeared to throw that plan into doubt when President Nicolas Sarkozy proposed, with Afghan President Hamid Karzai at his side and seemingly in agreement, that NATO end its mission in 2013 - one year earlier than planned.
Mr. Panetta is gathering with his European counterparts at a delicate time for NATO, not only because of the uncertainty surrounding the military mission in Afghanistan but also because of a growing gap in military power between the U.S. and nearly all other European members of the alliance.
That chasm is not expected to narrow even as the U.S. reduces its defense budget by nearly $490 billion over the coming decade and reduces the size of the Army and Marine Corps.
It announced last week that it will remove two Army brigades from Europe in the next two years, leaving one in Germany and one in Italy.
The alliance also is quietly discussing the possible withdrawal of American nuclear weapons from Europe in coming years. The nuclear issue is on the agenda for the Brussels meeting.
Gen. Ray Odierno, the Army chief of staff, said last week that the two brigades being removed from Europe will be eliminated rather than reassigned to U.S. bases. Both are based in Germany - the 172nd Infantry Brigade, in Grafenwoehr, and the 170th Infantry Brigade, in Baumholder.
Gen. Odierno said that, in the long run, this change will benefit both the United States and its European partners because U.S. Army combat and support units will periodically rotate in and out of Europe for training and joint exercises that are designed to meet the needs of the European forces.