Was america's essential foreign policy goal in the period 1812-1924 an essentially defensive one designed to protect its still fragile republican experiment against the dangers from reactionary european great powers and to isolate itself from european quarrels?

No, it was more of an aggressive, expansionist policy designed to guarantee that the United States would be the dominant power in all of North Americas, and possibly in Latin America. Thank you for posting your question. I hope that this answer helped you. Let me know if you need more help.