Why did the united states at first resist intervening in world war i? why did it later retreat from this policy and send troops?

The United States wasn't interested in an European conflict first and in the internal affairs the isolationist were stronger. The casus belli was the sank of the Lusitania by a German U-20 sub (Of the 197 Americans on board, 128 lost their lives.). But the main reason is The United States had close economic and political ties to Great Britain.

I'll go with that