Alright, it's true that Haig was easily one of the worst
commanders-in-chief during World War I, sending thousands of his
countrymen to death at battles of the Somme and Ypres.
But Petain, prior to his disgrace as a Nazi collaborator during
WWII, was actually one of the most outstanding generals of WWI. He
led the defence and succeeded in holding Verdun against the German
onslaught in one of the fiercest battles of the war, and he revived
French morale and fighting spirit after the mutiny of 1917 by
showing genuine concern for the welfare of his soldiers. His
subsequent collaboration with the Nazis had more to do with his
personality rather than any actual belief or endorsement of Nazi
ideology. Petain was a staunch patriot and possessed a somewhat
melancholic character, and his military skills were geared more
towards conservation and defence rather than attack and conquest.
When the Nazis broke the power of the French army with Operation
Sichelschnitt, Petain believed that the only way for the French
Republic to continue in existence was through collaboration with
Hitler, and hence he agreed to become the head of the Vichy. This I
believed stemmed more from his patriotism than from any belief in
Nazism, and so I think we should not see Petain in such a bad light
as some others have made him out to be.
If you are interested, you can try reading Corelli Barnett's
The Swordbearers, which
has a good deal of material on Petain during WWI.