The US isn't lacking launch capacity unless you're talking about humans. if you recall the pluto/KBO flyby mission "new horizons", that was launched from a us location on a us rocket, a delta iv, which also launched some other well known large recent payloads, such as the Mars Science Laboratory. The delta iv is a pretty capable launch vehicle, as are other smaller rockets in its family.
but that's pedantic because its obvious that you are talking about humans.
I suppose, I am not wholly sure what you desire for that state to do, however. The US could develop a parallel system to the Soyuz, but it would take an awful lot to make it as reliable as Soyuz, and perhaps more pertinent would be the question of what in the world is it for: probably if the US/Russia collaboration falls apart the entire future of the ISS would be drawn into question, and its possible the US would have no further desire to send crews there to begin with. that is, if the US uses access to soyuz, it probably also looses its use for a soyuz like vehicle.
As it stands the ISS is sort of questionably useful, I have a tendency to ramble so I shan't try to go into that.
if the US, or any state, is seeking to send humans to some meaningful destination, then its probably useful for that state to have means to send humans to space. as it stands neither the US nor any other country, with the possible exception of China, seem to be actively pursuing an expanded programme of human spaceflight to a destination apart from the ISS. The US is in its perpetual state of 'mars, soon' or 'moon again, soon'. But without a clear, achievable goal (which means, without a goal and the funding to get there) neither nasa nor anyone has much need of a system to launch humans into space.
considering the current state of NASA's budget, its probably more economical to pursue robotic projects for the foreseeable future.
Humans are expensive. they are periodically useful, and certainly some projects could incorporate human geologists, pilots, and other scientists to great effect in the role of astronauts. but as it stands, there are many places that have yet to be explored to the extent that's entierly achievable through robotic means.
to give a quick example: the outer planets of Uranus and Neptune, and their systems of satellites.
these were visited both briefly by the Voyager II probe after its flyby of Saturn. In the course of that miss
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