On allowing humanitarian aid to enter Gaza and otherwise "US military aid could be in jeopardy".
I noted their concern for civilians. Trump indicated he shares these concerns too and how bad it looks for Israel.
If it came to a public letter, what was said in private over the prior 12 months? When an ally resorts to a public letter, it's not the first conversation, and it's probably the mildest those threats have been articulated, because the US needs to be mindful of PR, too. Conversations behind closed doors don't have those PR concerns.
Also, note Trump's caviat in his concern. It looks bad for Israel. He doesn't share the same concerns, most certainly not on the same level as the current admin. He frames it as a PR issue, not a humanitarian issue or possible genocide.