Russia would consider using force if necessary to respond if the US put a combat weapon into space, according to a senior Russian official.
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And Russia would be right to do so.
Positioning orbital weapons platforms above the world's nations would be a provocation and, considering the US' often celebrated military superiority, a completely unnecessary move to provide for the national defense. Besides, as Woj pointed out, it's more likely the US will find itself bogged down guerrilla wars that spread like sunflowers instead of some high orbit battle of wits.
The Pentagon's plan suffers from an inescapable flaw, which would occur to a sci-fi literate 12 year old who'd read her Heinlein, Asimov or Pohl: regardless of the supposed sophistication of any American fielded platform (and given demonstrated American habits of development, it's a sure bet the Air Force will try to create a machine that can do everything and possess sufficient accuracy to kill a sparrow hiding under a bush in the middle of the night during a thunderstorm), its raw lethality could be easily matched by far simpler devices placed in orbit by Russia or China or India or whoever.
Also, destroying these clumsy terrors by deploying killer satellites that locate, orbit nearby then explode, irreparably damaging the weapons platform, would be a fairly trivial engineering task for many nations using current techniques. It's easy to imagine Russia developing, advertising and selling such devices on the open arms market for others to deploy and/or enhance further (China and India would almost surely be early adopters).
Of course, to answer this threat the Air Force would have to develop anti-killer satellite defense systems that placed even more destructive hardware in orbit requiring, in turn, other nations to field an answer to this.
The cost of such an orbital arms race would very quickly escalate to absurd and obscene levels, even for the world's 11 trillion dollar colossus.
There's an astounding amount of hubris on display here; an unwillingness to face the fact that the US military is having a difficult time dealing with the actual challenges its facing. So, rather than adjusting to new realities, better to return to Cold War grandiosity: to big dreams of killer machines that will make the world tremble.
But I have a feeling the world's almost done being impressed and growing very weary of trembling.
.d.