BOTH HAD LARGE IMPACT SOME HISTORIANS BELIEVE IT IS THE SAME WAR WITH A REST IN BETWEEN WWI AND WWII WIPED OUT GENERATIONS OF MEN
AnswerWhile WW2 certainly had more wide-ranging involvement and immediate impact on the globe, the prevailing view amongst military historians and political scientists is that World War One actually had the most far-reaching impact.
Practically all of the political conflicts and actual wars fought post-WW1 can be directly traced back to things that happened during (or at the conclusion of) that war. Here's a short list of things that happened during WW1 that had major impact on 20th-century life:
- Treaty of Versailles - the terms and conditions set forth in the treaty concluding WW1 are pretty much universally recognized to creating the conditions in Germany for the rise of National Socialism (and its attendant high priest, Adolf Hitler). The treaty forced a change in the German government, leaving the new government severely weakened and easy fodder for a radical nationalist movement.
- Centralized government - the "total war" concept and necessity of mobilizing the entire country's economy towards fighting a war caused an radical increase in the power of federal governments, as well as increasing their size several fold. Politically, WW1 effectively settled the issue of where the majority of power should be concentrated in any country. Federal (central) governments have only increased in size and power since WW1. Strong federal governments have largely dictated the course of most of the 20th century.
- Concept of Global Collective Security - ideas around the creation of a global community and its responsibility towards collective peace arise out of the chaos of WW1. While the League of Nations was a failure, the idea lives on in the United Nations and many of the global quasi-governmental organizations that regulate much of international disputes these days (e.g. World Court, the WTO, etc.)
- Communism as a valid form of government - the allowing of Lenin's famous "sealed train" to St. Petersburg in 1917 is perhaps the most outstandingly short-sighted disastrous action of the entire 20th century. Allowing Lenin the ability to participate in the Russian Revolution is a catastrophe of biblical proportions. This action cleared the way for Lenin to highjack the Russian Revolution (which was well on its way to leaving a democratic government in control of Russia) and then fight and win the Russian Civil War. WW1 thus led directly to the first establishment of a Communist regime. It then directly to all the Communist revolutions and conflicts elsewhere in the world (many of which are still ongoing). Without this action, it is highly unlikely that Communism would ever have been considered a possibly valid form of government by anyone, so there would be NO Communist countries anywhere.
- Rise of the United States as a global power - WW1 was the impetus that changed the American vision to be one of a global scope. Up until then, the United States was pretty much solely concerned with affairs in the Western Hemisphere (N/S America), and had all the characteristics of becoming an imperial power in the mold of Great Britain. WW1 radically reshaped the American view of the world (and the US's proper place in it), and indeed, changed Europe's view of the US, too. The U.S. emerged from the war as an industrial and political superpower, ahead of all other world powers. Without WW1, it is unlikely that the US would have achieved such a powerful position.
- Death of Empires - WW1 led directly to the death of the German, Austro-Hungarian, Ottoman, and French Empires, and indirectly to the death of the British Empire (by saddling Great Britain with a huge war debt, it made the maintenance of the Empire too expensive). The enormous cost in blood and money of WW1 left those countries unable to maintain the political and economic power needed to hold on to their extensive overseas colonies, which slowly began to break away (or, in the case of the losers, have them stripped away).
- Ideas of National Self-Determination - the end of WW1 began the promotion of the idea that all peoples should be empowered to determine their own government - that is, they should be able to form their own nation. The preceding Death of Empires accelerated this movement, but the idea was popularized at the end of the war. We are still seeing the effects of the influence of this idea, as more and more ethnic groups clamor for the right to form their own country, even those within larger established countries (e.g. Kurds, Chechnya, Basque). Indeed, most of the conflicts in the late 20th-century can be traced to the idea of ethnic self-determination.
- Proximate cause of the Great Depression - in particular, the crushing war debts owed by many countries greatly contributed to the worldwide depression of the 1930s. The conduct of Total War caused a radical change in most industrialized countries' economies, which then greatly influenced the poor economic behaviors which led to the Great Depression.
- Root cause of World War 2 - most historians now agree that without WW1, WW2 would have never happened. The rise of extreme nationalism of Germany, Italy, and Japan is directly traceable to WW1 events, and the isolationism and passivity of the US, France, and the U.K. is also attributable to their experience in WW1. National Socialism and Communism would not have existed, and thus one of the huge sources of tension would have disappeared, had WW1 not happened. So, pretty much anything that can be attributed to WW2, can thus be seen as a 2nd-order root cause of WW1.
- Validation of the Total War concept - WW1 was the first Total War in modern (industrial) times, where civilian infrastructure became valid military targets. This led to the widening of warfare (whereas it had been mostly restricted to those in the military and those immediately proximate to a battlefield) and the consequent horrific civilian casualties of all subsequent wars.
I'm sure you can come up with other examples, but the general thrust is this: without WW1 being fought, and being fought (and won) in the particular manner it was, virtually everything in the 20th century would be radically different. Indeed, most historians now see WW1 as the seminal event of the 20th century. Had it been handled better - most historians believe it was conducted extremely poorly, both from a military and a political viewpoint - it is likely that many, if not most, of the horrors of the 20th century could have been avoided. Certainly, no WW2, none of the horrors of Communism, probably no atomic bomb, no Indochina/Vietnam War, no Korean War, none of the incessant wars of the Middle East, no Holocaust, none of the incessant little guerrilla wars so common now. On the other hand, likely there would have been no Space Race, no nuclear power, no (much less prevalent) Women's Sufferage, much of the world would still be a colony of one of the Great Powers, and the great flowering of Western scientific advancement would be considerably less with no strong central governments (and no major wars) to push such a fast rate of technological innovation.