soft underbelly

plural soft underbellies

1. idiomaticA weak spot; a soft spot.

   But supply convoys are the soft underbelly of a powerful, modern military force that the Taliban is incapable of matching in conventional combat.

   2009 March 30, Mustafa Qadri, “Nato's soft underbelly”, in The Guardian

   2 more examples

   Scientists have known for years that the Thwaites glacier is the soft underbelly of the Antarctic ice sheet, and first found that it was unstable decades ago.

   2014 May 12, Suzanne Goldenberg, “Western Antarctic ice sheet collapse has already begun, scientists warn”, in The Guardian

   The current, predominantly right-wing perception is of the Balkans as a contagious disease, an infectious sore in the soft underbelly of Europe, best left to fester in isolation.

   2014, Christian Promitzer, Siegfried Gruber, Harald Heppner, editors, Southeast European Studies in a Globalizing World, LIT Verlag Münster, page 119