Why did some in the U.S. Senate oppose the Treaty of Versailles?

Answer 1

Here are three reasons Wilson failed to gain passage of Treaty of Versaille (with its League of Nations provision):

1) Wilson refused to accept ANY of the Senate's fourteen proposed "reservations"/modifications... with which it most certainly would have passed.

Henry Cabot Lodge told the story of the ratification vote and detailed some of his party's concerns in a book mentioned here:
http://156.33.195.33/reference/reference_item/Versailles.htm

Note especially this point:
"Senator Lodge, chairman of the Foreign Relations Committee and de facto majority leader, was troubled by the peace treaty, taking particular exception to Article Ten of the League Covenant, which he and others felt required all League members to come to the aid of any member state under attack."

To clarify: passing a treaty "with reservations" is not at all unusual. These would then have gone back to the other nations who were party to the treaty for their approval.

2) The President's choice of negotiating team - esp. failed to involve ANY Republican Senators in the treaty negotiations (even though they were the majority in the Senate, whose support he needed for ratification)

It was not an issue of simply "offending" people (by leaving them out), but that by leaving them out Wilson failed to make sure their CONCERNS about specifics (such as that reflected in #1 above) were addressed. It is also arguable that such a team would have had a bit more saavy to negotiate more strongly and gain a treaty that better protected AMERICAN foreign interests.

3) Wilson 'burnt himself out' with his travels around the country to drum up support, so was weakened, less able to engage in constructive discussions with the Senate (This activity, against expert advice, may well have contributed to his dibiliating stroke... it is possible, however, that underlying health issues help explain his stubbornness at this time.)

Answer 2

The Treaty of Versailles is the Paris Peace Conference after World War I organized by the winners to settle unresolved issues. Then US President Woodrow Wilson pushed for the Fourteen Points program. He made the statement before Congress with the hope to "rally liberal opinion throughout the world with his address, but his opening remarks were also designed as a sympathetic response to the new Bolshevik leaders in Russia, who had called upon Russia's western Allies to begin peace negotiations on a program of no annexations, no indemnities."
The Bolsheviks are the fore-runners of communist Russia so it is obvious that not everyone in Congress would agree with this sympathy.