With a conflict this large, I'd be surprised if any foreign powers of note didn't at least try to plant sources within all major factions, as well as picking their favorites and supporting them with financial aid and weapons. That doesn't mean that everything was completely orchestrated, just that foreign involvement was pretty much inevitable as soon as things got violent.
Their goal was to establish an independent Basque nation by forcing the French and Spanish governments to give up their respective territories.
Now, for a number of reasons, they haven't been particularly successful. Their attacks mostly involved bombings, robberies, kidnapping or plain old executions. They've used some military grade weapons like mortars but never had much resembling a full army, let alone tanks.
If things had been different and the US would have had an interest in having the CIA fund, train and equip the ETA to the point where the ETA would have been able to wage an actual war of secession against Spain and/or France, wouldn't you agree that this situation would have been drastically different from "merely" killing less than 1000 people over decades of activity?
Yet apparently by your standards because ETA still existed and killed people without the hypothetical CIA support the situation would have been no different in the grand scheme of things and the US's involvement would have been completely irrelevant.
There are plenty of terrorists or separatists or nationalists or rebels or freedom fighters in all kinds of countries and plenty of them have killed people. Yes, people kill people without covert (or overt) military support from the US and other powers but pretending that makes the level of support they receive irrelevant is absurd.
Without US involvement, the Syrian civil war would look quite different, just like the Ukrainian civil war would look very different without Russian involvement.