What were the nations during World War One?
I assume that you are asking what countries fought in World War I.
There were two groups of allies in WWI. We now refer to them as the Allied Powers (or Allies) and the Central Powers. The first of these groups is sometimes called the Triple Entente while the second is sometimes called the Triple Alliance.
At the beginning of the war, the most important countries on the Allied side were France, the United Kingdom (at the time, this included Canada, South Africa, Australia, and New Zealand), and Russia. The most important countries on the side of the Central Powers were Germany, Austria-Hungary, and the Ottoman Empire. Later in the war, Russia dropped out and the United States entered the war on the Allied side. These were the most important countries in the war.
Many other countries were affiliated to some degree with the Allies in WWI. Italy and Japan are also seen as major powers who eventually joined the Allies, though Japan’s involvement was minimal. Serbia was not a major power, but it was involved in the war early on and suffered major losses. The UK and France had many colonies that sent men to help with the war effort, but those countries were not involved of their own free will. Many other minor countries were officially on the Allied side in the war. These include Brazil, Portugal, Romania, Greece, Siam (now Thailand), and China. In this link, you can see how many casualties each of these countries suffered in the war so you can decide which countries were major combatants.