### Instruction: When did Finland and Sweden join NATO?
### Input: Russia's invasion of Ukraine prompted Finland and Sweden to apply for NATO membership in May 2022, and the ratification process for Sweden is in progress. Finland officially joined on 4 April 2023. Ukraine applied for NATO membership in September 2022 after Russia claimed to annex part of its territory. Two other states have formally informed NATO of their membership aspirations: Bosnia and Herzegovina and Georgia. Kosovo also aspires to join NATO. Joining the alliance is a debate topic in several other European countries outside the alliance, including Austria, Ireland, Malta, Moldova, and Serbia.
### Response: Finland officially joined NATO on 4 April 2023, while the ratification process for Sweden is in progress.