How can a source explain why US senators rejected the Treaty of Versailles?

A source can help explain why senators rejected the Treaty of Versailles by showing what prevented agreement. First identify what the source actually says, then connect each detail to a reason. Focus on that evidence rather than everything you know about the treaty.

Consider the US Senate Historical Office's account of the unsuccessful vote on 19 November 1919. It describes President Woodrow Wilson's relationship with Republican senator Henry Cabot Lodge, who chaired the Foreign Relations Committee. This is a retrospective account, meaning it was written after the events, not a speech delivered by Lodge in 1919.

You can draw out three separate arguments:

Notice why these are three points: they concern the relationship between individuals, the negotiating process and the terms of approval. Writing 'Lodge disliked Wilson', 'Wilson disliked Lodge' and 'they argued' would mostly repeat one reason.

You can use this method with other sources: source detail → inference → relevance to the question. A detail is the evidence; an inference is the conclusion you draw from it. Do not claim more than the evidence supports. Excluding senators can support an argument about resentment, but cannot by itself prove how every senator voted or what each privately believed.

For a broader historical explanation, this short account is incomplete. Republican opponents included both senators willing to accept a treaty with reservations and those rejecting League membership more fundamentally. Wilson also influenced Democratic opposition to a reserved treaty. The Senate's account of the Irreconcilables highlights the second group. Those positions must be distinguished rather than treating all Republican opposition as identical.

For a source-based response, however, use three reasons that the supplied text supports. Outside knowledge may help explain the background, but keep it separate from evidence taken from that source.

Have a go at the Senate-source exercise in the First World War peace-settlement questions. Find three different reasons for rejection, then check that each has its own supporting detail rather than repeating the same argument.