Alliance of powers (4)
I believe the answer is:
axis
I believe this is a double definition.
'alliance' is the first definition.
(alliance between two countries)
'powers' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for axis that I've seen before include "Imaginary centre of rotation" , "Imaginary central line" , "Imaginary line about which a body rotates and a Dublin theatre" , "One of very elliptical orbit" , "Line of symmetry" .)