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Those who coined the phrase "World War II" applied it to that war which started between those four nations and their global empires when Germany invaded Poland 1 September 1939. And it stuck. And it became accepted as such. It can also be rejected as such.


EDIT:Commentary This Question has generated a variety of both Answers and Comments which express many differing opinions (granted, many backed with

While Questions that ask "reasonsWhy..., but still" often invite opinions), as to when the war started, who was involved at what point, how many were involved and when, at what time did the various regional conflicts cease being separate wars and become absorbed into the "world" war, etc. These are all being vigorously presented and defended and the traffic generated for this site is a good thing. I am not advocating closing this Question from the standpoint that it requires opinion-based answers. I think this kind of post(and some good ones have been expressed here in some Answers and the ensuing discussion is healthy for History:SE.

What I am suggesting though is thatComments), this Question can be addressed from a marginally historic perspective, and that is by examining the historical use of the term World War II, when it came into use and by whom, what are its roots in previous usage for other global conflicts (etc.) - hence why I chose to post this Answer from that standpoint. I think this approach can keep the Question grounded in terms that are not primarily opinion-based (even though use of the term may well be challenged as opinion- or politically-based) in hopes that by doing so this Question can remain open for years to come with many more great Answers and Comments added (which at some future date others may refer back to for historical perspective by the way) without fear of having this closed for being opinion-based.

Ok... down off my soap box now. (I may remove this Edit later, just adding it here for visibility and consideration for a while.) Cheers to all! :-)

Those who coined the phrase "World War II" applied it to that war which started between those four nations and their global empires when Germany invaded Poland 1 September 1939. And it stuck. And it became accepted as such. It can also be rejected as such.


EDIT: This Question has generated a variety of both Answers and Comments which express many differing opinions (granted, many backed with reasons, but still opinions), as to when the war started, who was involved at what point, how many were involved and when, at what time did the various regional conflicts cease being separate wars and become absorbed into the "world" war, etc. These are all being vigorously presented and defended and the traffic generated for this site is a good thing. I am not advocating closing this Question from the standpoint that it requires opinion-based answers. I think this kind of post and the ensuing discussion is healthy for History:SE.

What I am suggesting though is that this Question can be addressed from a marginally historic perspective, and that is by examining the historical use of the term World War II, when it came into use and by whom, what are its roots in previous usage for other global conflicts (etc.) - hence why I chose to post this Answer from that standpoint. I think this approach can keep the Question grounded in terms that are not primarily opinion-based (even though use of the term may well be challenged as opinion- or politically-based) in hopes that by doing so this Question can remain open for years to come with many more great Answers and Comments added (which at some future date others may refer back to for historical perspective by the way) without fear of having this closed for being opinion-based.

Ok... down off my soap box now. (I may remove this Edit later, just adding it here for visibility and consideration for a while.) Cheers to all! :-)

Those who coined the phrase "World War II" applied it to that war which started between those four nations and their global empires when Germany invaded Poland 1 September 1939. And it stuck. And it became accepted as such. It can also be rejected as such.


Commentary

While Questions that ask "Why..." often invite opinions (and some good ones have been expressed here in some Answers and Comments), this Question can be addressed from a marginally historic perspective, and that is by examining the historical use of the term World War II, when it came into use and by whom, what are its roots in previous usage for other global conflicts (etc.) - hence why I chose to post this Answer from that standpoint. I think this approach can keep the Question grounded in terms that are not primarily opinion-based (even though use of the term may well be challenged as opinion- or politically-based) in hopes that by doing so this Question can remain open for years to come with many more great Answers and Comments added (which at some future date others may refer back to for historical perspective by the way) without fear of having this closed for being opinion-based.

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The various definitions of the term itself include no objective criteria (e.g. 60% of nations of the globe, or 60% of the land-mass of the globe, etc.) but rather subjective criteria. For example, if using Merriam-Webster's definition (my emphasis added):

a war engaged in by allall or mostmost of the principalprincipal nations of the world

The various definitions of the term itself include no objective criteria (e.g. 60% of nations of the globe, or 60% of the land-mass of the globe, etc.) but rather subjective criteria. For example, if using Merriam-Webster's definition:

a war engaged in by all or most of the principal nations of the world

The various definitions of the term itself include no objective criteria (e.g. 60% of nations of the globe, or 60% of the land-mass of the globe, etc.) but rather subjective criteria. For example, if using Merriam-Webster's definition (my emphasis added):

a war engaged in by all or most of the principal nations of the world

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Many good answers here already - each with valid points to consider, and any one of which I could accept if I had posted the question (and a +1 from me on more than one of them). However, the definition of World War has not yet been addressed (i.e. when is it appropriate to use the term World War in connection with an international conflict?).  


Definition


Origins


Accepted Usage


Specific Case: Start of WWII

Many good answers here already - each with valid points to consider, and any one of which I could accept if I had posted the question (and a +1 from me on more than one of them). However, the definition of World War has not yet been addressed (i.e. when is it appropriate to use the term World War in connection with an international conflict?).  

Many good answers here already - each with valid points to consider, and any one of which I could accept if I had posted the question (and a +1 from me on more than one of them). However, the definition of World War has not yet been addressed (i.e. when is it appropriate to use the term World War in connection with an international conflict?).


Definition


Origins


Accepted Usage


Specific Case: Start of WWII

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14 clarifications added on usage of term being applied due to technical state of war 1 September 1939, not based on the following declarations of war on 3 September 1939
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