Over the course of a five year war in Yemen, Saudi Arabia has killed the vast majority of civilians. The killings were overwhelmingly from airstrikes, which brings uncomfortable attention to the US.
The Saudi airstrikes, after all, are done almost exclusively in US-made warplanes, and dropping largely US-made munitions. The US has also, for much of the war, refueled the warplanes in-air to keep them going.
US officials have been concerned about their culpability in Saudi war crimes for some time. Not enough to actually withdraw from the war, but worried enough that they sent advisers to Riyadh to try to consult to reduce casualties.
Those advisers have largely failed to reduce the rate of casualties from airstrikes, and while the Pentagon has tried to claim the US is not in the “kill chain,” they’ve still manufactured the weapons that make up the chain itself.
That continued problem is driving Congress to try to limit US involvement in the war, and to cut arms sales to the Saudis. So far, Congress has enjoyed mixed results, and some are predicting President Trump will use a loophole to rush through delivery of more bombs to the Saudis.