Why was France granted an equal status among victors of World War II?

My opinion (again: opinion) is that it was unimaginable not to honor France as a victor of the WW1. After winning the Great War France was a top Power in the world, even if we call it now something as a paper tiger. Although Germany conquered France relatively faster than Poland (note Soviet assistance against Poland, which could not be compared to Italian help in French campaign of 1940), it could have been unfair as the fame of the previous war.

Churchills triumph, a novel by Michael Dobbs gives the best explanation for this. With Poland and Eastern Germany lost to Russia, France had to be built up as a bulwark against communism

It was France and Britain who originally declared war on Germany for the invasion of Poland.

For two years the United States sat on its hands and wanted nothing to do with the war in Europe. One might equally ask what gave USA the right to sit at the table, since USA only got involved after the attack on Pearl Harbour?

My personal opinion is that the main contribution of France was that they were not reluctant to declare war on Germany after they attacked Poland. In fact, France could easily just ignore the Germany's attack. I think France was rewarded so not to discourage such behavior.

France was a world power in 1945, as pointed out by Anixx's colonial map, and still is (after a fashion), today. Along with the three defeated powers (Germany, Italy, and Japan), it is a member of the G-7, which also includes the United States, Britain and Canada (and does not include Russia or China). This is true even though the war basically reduced France (and Britain) from "Great Power" to mid-level status.

France (and Britain) were also in World War II (along with Poland) from the very beginning, at least of European hostilities, (September, 1939). After a fashion, so was Russia (albeit on the "wrong" side in 1939). And China's involvement in fighting pre-dates the European war. In terms of "time served," the U.S. had the weakest claim to the winner's circle, except for the fact that she was so instrumental in the victory.

Another factor that may have affected France's status was her role in World War I. The Treaty of Versailles had been signed in France, and had been largely constructed for her benefit.

From Wikipedia, you can found this:

Il fut, dès l'origine, composé de cinq membres permanents, les États-Unis, l'Union soviétique, le Royaume-Uni, la France et la République de Chine, à la fois, parce que ce sont les principaux vainqueurs de la Seconde Guerre mondiale, et parce que représentant à ce moment-là la majorité de la population mondiale (en comptant les empires coloniaux), chacun à peu près à égalité.

Which can be translated into:

It was [the Council] composed, from the beginning, of five permanent members, the USA, the USSR, the UK, France and the Republic of China, both because those are the main winners of the Second World War, and because they represent the most part of the World's population (with the colonial empires), each with roughly the same amount.

According to this page, France and the UK were the only main remaining countries to be part of the League of Nations (organization that preceded the UN). Vichy's government was willing to get out of this league (the article said that this was maybe because of Germany's pressure). France was then suspended from the League, until the recognition of the French Committee of National Liberation in October 23th of 1944 (so after the beginning of the negotiations about the UN), France became a candidate to be part of that organization.

So, even though De Gaulle was not invited at Yalta, France's presence in the Security Council was supported by the UK (from the previous website: “il fut alors décidé, sur l'insistance des Britanniques, qu'un siège permanent serait réservé à la France”).

At the Conference of San Francisco (04/25 - 06/25 of 1945), France was already recognized as “future permanent member of the Security Council”, during that conference, France obtain the right to be part of the negotiation (thanks to that status of future member) and gave its best to add French as one of the working languages of the UN (this initiative was supported by Canada and most of the countries of Latin-America).

Here is the map of colonial possessions by 1945:

The French possessions are in blue.

As you can see, France controlled a territory comparable to the US, USSR, British and Chinese. Adding them meant adding representatives of a large portion of the world's population to the security council.

The main reason for the status of France after the WW2 was Churchill's position.

1. Soft power and colonies are important, but nothing prevented the US & SU (both with strong anti-colonialist sentiment) from breaking up the French colonial empire.

2. The war contribution of France was mostly in denying Germany the use of the French navy. The rest (resistance, Normandie-Neimen &c) is easily balanced by the Vichy France collaboration.

What did matter was that Churchill remembered that the US became highly isolationist after WW1 (even refusing the ratify the Versailles treaty), and his worst fear was that Britain will be left to face the USSR alone in Europe after the US left. So he wanted to preserve France as a Great Power to help Britain counter the SU. (Churchill says so explicitly in his WW2 book).

Another reason for Churchill's opposition to disbanding the French empire was that it would have destabilized the British one. (I don't think Churchill says so explicitly, but, given his unwillingness to "preside over the dissolution of the British Empire", I think this is a reasonable assumption).

Thus Churchill exchanged his support for Roosevelt's pet project - making China one of the Great Powers - for Roosevelt's support for keeping France one of them.

Stalin supported both pet projects in order to dilute the Anglo-Saxon power. He correctly bet that France will never forgive the humiliation of being liberated - and France worked hard to show its independence from the British and Americans, culminating in leaving the NATO military organization.

Chiang Kai-shek had a Russian daughter in law, also he was weakened by the Communist guerillas (and thus could be controlled), so Chinese inclusion in the Great Powers was in Stalin's interests too.