If WW2 were separated in phases, what would they be?

Answers to this will be pretty subjective, and they will also be influenced by what one's definitions of the Second World War actually are. For example, do you include the Winter War? Do you include Second Sino-Japanese War?

However, most historians doing broad works on the whole war would likely characterize the war into three or four parts, and those parts are largely defined by the level of success the Axis were having. So, for example, you could define the first part of the war as Axis success, with Poland, France, most of North Africa, and a large portion of European Russia being occupied by December of 1941, then the tide turning in 1942 to mid 1943, then a drive to victory by the Allies from mid 1943 to 1945. Alternatively, you could (and historians frequently do) add in a phase of low intensity fighting for the 'Phony War'. You could also add in a 'Pre-War' phase, and this for many historians often includes the conflict in China.

World War II is often characterized in terms of the 'turning points' by historians, particularly those writing more accessible histories. Often, these 'turning points' make up the breakpoint between multiple books in a series even. For Europe, this is often identified as Stalingrad (Fall 1942 to late Winter, 1943), while in the Pacific it is often identified with Midway and Coral Sea (Late Spring to Summer, 1942). While Axis offensives continued after these setbacks in both Theatres, they represent the high water mark of Axis military strength.