(Reuters) – Finland’s President Sauli Niinisto and Prime Minister Sanna Marin said on Thursday their country must apply to join the NATO military alliance “without delay”, a major policy shift triggered by Russia’s invasion of Ukraine. Sweden’s ruling Social Democrats are expected to decide on Sunday whether to overturn decades of opposition to NATO membership, a move that would almost certainly lead to Sweden also asking to join the 30-nation alliance. The two countries had for decades held the belief that peace was best kept by not publicly choosing sides. Any accession process is expected t…