The only reason we are getting involved is because the government wants to get involved. The only way we know the "red line" was crossed is because we were told it was crossed. The government could have easily made a rational case that 150 possibles out of 90000 deaths was not conclusive. Because it isn't conclusive.
The government wants to be involved in this war and not on ethical grounds, because they guys we are supporting are terrible, terrible human beings. Why do they want to be involved? I couldn't say and they won't say. I can only speculate. I can, however, reject their clearly illogical public rationale. It is not ethics. It is not to stop biological warfare. It is not to promote freedom or protect people. It is something else.