It's a huge oversimplification. What exactly did the Obama administration do to destabilize the Assad regime in the first place? This all came out of years and years of corruption, oppression and mismanagement by the Assad family. This came out of the organic political movement of the Arab spring. If we can fault one big decision, maybe you can point to the Iraq War, but Obama didn't create this situation in any meaningful way.
Next, how did our involvement abet the rise of ISIS? This statement:
We trained and armed jihadist rebels in Syria, something that Paul was strongly opposed to.
is certainly wrong. The US did not arm ISIS, we sluggishly aided the FSA mostly with CIA advisors and non-lethal aid. We also ended that program when ISIS began to truly expand and we had the concern that our aid could fall into their hands. Some people just want to paint all the anti-Assad forces as Islamic radicals, but that's just not factual.
I think it's actually more likely that we gave ISIS a chance to fester because we didn't intervene with enough gusto at the beginning. Obviously, politics played a huge role there with Obama's reelection creating an air of restraint in committing to any large role in brining down Assad. But, you could make the argument that we were actually too timid as an international community and too willing to allow this disaster to continue. If we had gotten behind the FSA with full force we might have a unified transitional government in Syria instead of the 30 years war style failed state we see right now.
This debate just reflects the provincial worldview of most Americans. We always see America as the crucial player, either because we're intervening too much or not enough. Here, I don't think there are any easy answers and anyone who is peddling them is not to be trusted.