If China or the USA made Bitcoin transactions illegal the price would probably plummet. How far is anyone's guess, but below $1000 would be a sure bet in my opinion. Over time it would rise again but if major exchanges were forced to close near-term trading activity would drop off sharply.
Granted this is a highly unlikely scenario and many people already successfully use Bitcoin to circumvent laws, but given the recent ransomware attacks and increased scrutiny of exchanges inevitably some out of touch politicians will propose a Bitcoin ban. That said, blockchain analysis is still far too expensive for the government to track down everyone's BTC (especially given the preponderance of quality tumbling services). So any ban would be useless towards stopping BTC activity should the core developers stick to their guns, so to speak.
RE: What can cause Bitcoin to crash?