Recently there has been talk of the US arming certain governments or organizations to support its interests, but from what I’ve seen supporting one side of a civil war with arms and training tends to have unintended consequences in the long term.
Even the proxy wars that did succeed in the short term, such as An example of this is the US arming and training of the Mujaheddin in Afghanistan in the 80s, which led to a civil war in which the Taliban took control and then oppressed its people and created a terrorist haven.
(Edit: I realized now that I misphrased this. I wrote this quickly and didn't mean to say that all American proxy wars were like this specific event.)
Has the US ever meddled in an already contentious region and achieved all of its goals? If so, what made this case successful as opposed to other failures?
This is a major leading question and not a very good one because you've come up with an answer before examining the evidence and just want confirmation from others. Try something like picking a sample of a few "proxy wars" (however you want to define that) and then asking about the effects they had in turn. Then you can examine the evidence for each individually and likely come to a much more level-headed conclusion and maybe even challenge some of your own pre-conceived notions, which is a good thing because it means you're learning.
You are sayings several different things which may all be true, so I'll try to dissect your post a little.
but from what I've seen supporting one side of a civil war with arms and training tends to have unintended consequences in the long term.
I think that this is true and largely inevitable when speaking about war and revolutions. It is very difficult (I’d say impossible) to determine the outcome of supporting a proxy war 50 or 100 years down the line. So yes, long term unintended consequences happen, but that doesn't mean bad per say.
Even the proxy wars that did succeed in the short term, such as the US arming and training of the Mujaheddin in Afghanistan in the 80s, led to a civil war in which the Taliban took control and then oppressed its people and created a terrorist haven.
I’ll agree with your premise if only to move to the last question.
Has the US ever meddled in an already contentious region and achieved all of its goals?
Afghanistan in the 1980s? First we have to determine what the goals were. If the goal of arming the Mujaheddin was an effort to achieve a Soviet withdrawal, then the US did achieve its goal. Perhaps a goal was strengthening the intelligence services of Pakistan, a potential ally against Iran. Again, success. That these things later turned out against US interested doesn't mean that the proxy war didn't achieve success at the time.
As for an example of us ‘meddling’ in a proxy war that led to an achievement of all it’s goals and little negative unintended consequences I’d say US support of the Greek government during the Greek Civil War.
What about WW1 and WW2 (European front)? I would also say that the American support in Israel during Yom-Kipur war and ever since proved very successful in terms of building a strong and stable ally.
Kurdish regional government is a pretty stable "proxy"/ ally-country of the USA. It is stabilizing the region and is aligned with US/ Western interests in the region.
No.