Answer: sweeping blows
Step-by-step explanation:
"Sweeping" has multiple meanings, one of which is to infer the intensity or wide ranging nature of an initiative. For instance, if sweeping reforms are introduced in an industry, it means that the reforms are wide ranging and significant.
A "blow" is synonymous with a setback so in using the phrase, "sweeping blow", the author wanted to show just how intensive the setback or tragedy that the American Indians had suffered was.