NATO Secretary-General Jens Stoltenberg (L) and Swedish Prime Minister Ulf Kristersson (Photo: Bloomberg)

Swedish Prime Minister Ulf Kristersson handed over the final documentation to the US government on Thursday, the last step in a drawn-out process to secure the backing of all NATO members. Sweden and Finland, which shares a 1,300-kilometer border with Russia, are the most significant additions to NATO in decades.

Sweden will benefit from the alliance's common defense guarantee under which an attack on one member is regarded as an attack on all.

While Finland joined the alliance last year, Sweden was kept waiting by Turkey and Hungary. Turkey approved Sweden's application in January. Hungary delayed its decision on Sweden's accession until Kristersson made a goodwill visit to Budapest in February.