By Sabine Siebold, Robin Emmott and Pavel Polityuk MADRID/KYIV (Reuters) – NATO on Wednesday branded Russia the most “direct threat” to allied security after Moscow’s invasion of Ukraine and vowed to modernise Kyiv’s beleaguered armed forces, saying it stood four-square behind Ukrainians’ “heroic defence of their country”. Completing a summit dominated by the invasion and the geopolitical upheaval it has prompted, NATO formally invited Sweden and Finland to join the alliance and pledged to reinforce combat-ready and rapid-reaction forces on its eastern flank, closest to Russia. President Joe B…