Question

Representatives of every Allied nation except which country attended the peace conference in Paris?

Answer 1

Russia.

Explanation:

Since the beginning of WWI (1914-1918), the Russian Empire had fought the Central Powers in alliance with England and France.

As a result of many defeats suffered in battle against the Germans and as result of revolution (February 1917 and October 1917 revolutions) the Russian Empire was ended and the Tzarist government (the main members of the Russian Imperial Family, the Romanovs, were killed on July 6, 1918) was succeded by a Bolshevik government led by Lenin.

In accord with the promises of the Bolsheviks, Trotski signed, on 3 March 1918, in the name of the Russian government, a separate peace Treaty (Brest-Litovski) with the Central Powers, by which Russia renounced all territorial claims in Finland, the future Baltic States (Estonia, Latvia, Lithuania), Belarus and Ukraine.

Immediately after the Communist Take-Over of December 1917 (November 1917 by the Julian Calendar), Russia was engulfed in Civil War (Red versus White Army, that lasted until 1921) in which the Western Powers financed the opponents of the Bolshevik because they were worried about (1) a possible Russo-German alliance, (2) the prospect of the Bolsheviks making good on their threats to default on Imperial Russia's massive foreign loans and (3) that the Communist revolutionary ideas would spread (a concern shared by many Central Powers).

https://en.wikipedia.org/wiki/Russian_Civil_War

So the reason because the Allied didn't invite the Bolshevik government to Paris Peace Conference (1919-1920) was because they refused to recognize the new Bolshevik government and some other reasons for this were that the Bolshevik decided to repudiate Russian outstanding debts to the Allies and to publish the texts of secrets agreements between the Allies concerning the postwar period.

http://2001-2009.state.gov/r/pa/ho/time/wwi/89875.htm

In 1919, the Big Four met in Paris to negotiate the Treaty: Lloyd George of Britain, Vittorio Emanuele Orlando of Italy, Georges Clemenceau of France, and Woodrow Wilson of the U.S.