As soon as you open international borders and ease lockdowns, regions and countries with few cases will see cases rise. We got to our current situation with just a handful of imported cases. Now that there are millions of carriers, you can't possibly hope to contain it with people traveling everywhere for work and leisure. Quarantining incoming travelers isn't feasible, at least not without sacrificing international business and leisure travel completely (a huge chunk of the GDP).
This is a siege where the castle only has food for two months. Unless you expect reinforcements (i.e. a vaccine or cure), extending the siege mostly means starving to death.