In WWI I see that we have Central Powers and Allies. Both are groups of countries who fight together against the other group.
In WWII we again have Allies and Axis.
I can't understand why do we call a group allies, while both groups are composed of allies.
First, ally (n. and v.) comes from the Old French verb:
Etymology: < Anglo-Norman and Old French, Middle French alier, Anglo-Norman and Middle French allier, alyer, allyer (French allier ) (reflexive) to unite oneself or itself to (a person, group of people, or thing) (c1100), to unite, combine, or join in kinship, friendship, association, etc
Thus the people who were fighting on the English-speaking side were "allies" - they were united:
2. a. A person who helps or cooperates with another; a supporter, an associate; a friend.
2.b. A person, state, military force, etc., united or associated with another by league or formal treaty, esp. for political or military purposes.
In WW2, we had Britain and its allies, and Germany and its allies. For convenience, the English speaking British used "allies" to refer to their friends. But there was a need to distinguish Britain and its allies, from Germany and its allies - the Axis powers.
Q: So why "Axis"?
A: It comes from an obsolete meaning of the word, "axis" which is related to "axle."
4.a. figurative. A central prop, which sustains any system (as [the god] Atlas was believed to support the revolving heavens). Obsolete.
1646 Sir T. Browne Pseudodoxia Epidemica 94 The Atlas or maine axis, which supported this opinion, was daily experience.
It then became a figurative use:
4.b. figurative. The relation between countries regarded as a common pivot on which they revolve; esp. the political association of 1936 (becoming in 1939 a military alliance) formed between Italy and Germany; later extended to that between Germany, Italy, and Japan; still later to that between other allied countries. Often used attributively, as Axis forces, Axis powers, and elliptically for such phrases, with consequent plural agreement. Also transferred, of any comparable association, or connecting common interest.
1936 Times 3 Nov. 15/1 The ‘Rome–Berlin axis’ is a conceit which has its momentary attractions.
1952 Economist 19 July 145/1 The Moscow–Peking axis.
1959 New Statesman 7 Feb. 177/1 The term ‘axis’ is looked on with disfavour here [i.e. in Bonn] as a reminder of the Berlin–Rome–Tokyo axis of the Nazis.