[Manthink]

Commonwealth don't need to commit troops whereas NATO needs.  Heng ah

PM Lee requested to stand in the middle
Or becoz he's too tall to stand behind or at the side.

The good news is unlike NATO, the Commonwealth isn't a military club...and nor does it need to make new enemies for justify its existence. 

That is why when PM Lee saw how NATO expanded and yet War still happen in Europe...So he decided it is best in SG's interest to follow the peaceful rise path as Asia Pac had been for last 4 decades...instead of following NATO's way of expansion and WAR making.

Since the fall of USSR,  Asia Pacific grew and prospered in peace and cooperation, Europe choose a different path of expansionism.