As American President Joe Biden commences his three-nation European tour, this week will be crucial for the Western alliance arraigned against Russia in the wake of the war in Ukraine. Several contentious issues will feature in the leadup to and during a meeting of the North Atlantic Treaty Organisation in Vilnius, Lithuania, prominent among which will be efforts to persuade Turkey not to block Sweden’s entry to the alliance, as well as discussions on Ukraine’s admission.

Turkey and Hungary have both opposed Sweden’s entry; the former argues that Stockholm has not done enough to curb the activities of those it terms Kurdish militants and the latter says it will take its cue from Ankara. Mr. Biden attempted to persuade Turkish President Recep Tayyip Erdogan to change his mind over a telephone call as did British Prime Minister Rishi Sunak. Both were apparently rebuffed. Swedish Prime Minister Ulf Kristersson is meeting Mr. Erdogan in a last-ditch effort to salvage the situation, and Mr. Biden will hope to persuade the Turkish leader when they meet in Vilnius.

Many Western leaders believe that Mr. Erdogan is acting at Russia’s behest to create dissension in Nato ranks, and Sweden’s admission therefore acquires considerable importance. The other issues that will feature in discussion are Ukraine’s admission and America’s supply of cluster bombs to Kyiv. While European nations are agreed that Ukraine cannot be admitted into the military alliance while the war rages, for that will mean elevating the conflict to one between Nato and Russia, there is pressure to recast the relationship in ways that show commitment to the cause of admission once hostilities cease.

This is a tricky subject and one that will need to be tackled with considerable tact. Ukraine’s president has sought a unified signal from Nato, asking it to show it is not afraid of Russia, and sought “clear security guarantees”. Some alliance members though believe it is more important to supply arms and ammunition to Ukraine, and not to take the provocative step of facilitating Kyiv’s admission to Nato.

The other contentious issue, of course, is the American supply of cluster bombs to Ukraine. Many European nations, including the United Kingdom, are signatories to an international convention banning the use of such munitions, and Washington’s decision is bound to feature in talks at Vilnius. Mr. Sunak is already on record as saying his country discourages the use of cluster bombs, but the Americans believe this objection is perfunctory rather than substantive. But other European countries may not be as sanguine or accommodating, and Mr. Biden may have some hard selling to do.