What did Senator Pittman believe was the reason for Republican opposition to joining the League of Nations?
I assume that you are asking about Senator Pittman of Nevada who gave a speech that is sometimes excerpted in primary document collections.
In this excerpt, Pittman does not come right out and say why he thinks the Republicans are opposing entry into the League. However, he implies two things. First, he implies that the Republicans are opposing the League simply for political gain. He implies that Republicans who are voting against it are putting the good of their party before that of their country. Second, he implies that they may be doing this for selfish reasons. He says that the refusal to enter the League would lead to the adoption of a policy of isolation and profit. He mentions selfishness and greed. He seems to be implying that the Republicans might make money off of wars and would therefore not like the League to bring peace.