On 1 July, as part of Task Force (TF) 19, Buck got underway for NS Argentia, Newfoundland, where it joined a convoy carrying US's 1st Provisional Marine Brigade to Reykjavík, Iceland.

As a convoy escort warship, Buck screened ships from enemy attack, pursued unidentified surface and underwater contacts and shepherded merchantmen to keep them in formation while underway.

When the port propeller fell off a few hours later, leaving the destroyer helpless, the fantail was cut loose since wave action was battering and chafing the hull.

On 3 August, while escorting a convoy of six cargo ships from Sicily to Algeria, Buck spotted the Italian submarine Argento making a reconnaissance patrol off the Sicilian coast.

After escorting a convoy back to the United States, the destroyer returned to the Mediterranean in late September 1943 in support of Operation Avalanche, the landings at Salerno, Italy.

While on patrol off Salerno, Italy, on 9 October, Buck was ambushed just after midnight by German submarine U-616 commanded by Siegfried Koitschka and hit forward starboard by at least one and possibly two torpedoes.