Don’t know that they could do much about a killer asteroid anyway.
A Tunguska sized piece might not be spotted until it’s minutes away. Not much time to warn people to duck!
Actually a lot - if they had enough of a lead time, and the political will - but given all the squabbling in Congress, your right - no amount of lead time measured in less than 100s of years would be enough. So no, a killer astroid would be just that: ELE.
Maybe the Chinese would be able to do something (certainly not the US) but who knows?