Force 10 from Navarone

In 1943, after their successful mission on the Greek island of Navarone, Major Keith Mallory (Robert Shaw) and Sergeant John Miller (Edward Fox) are sent to find and kill Nicolai. Originally thought to be a traitor who informed the Germans about Miller and Mallory during the Navarone mission, Nicolai is now known to be Colonel von Ingorslebon, a dedicated German spy believed to have infiltrated the Yugoslav Partisans as "Captain Lescovar" (Franco Nero).
To get to Yugoslavia, the two men pair with "Force 10", an American sabotage unit, led by Colonel Mike Barnsby (Harrison Ford), which has its own mission there. To maximise security, Force 10 steals a Lancaster bomber from a British air base in Termoli, Italy, rather than requisitioning one; but they are spotted by Military Police MPs before they can take off. Weaver (Carl Weathers), a US Army sergeant under arrest by the MPs, joins with Force 10 and forces his way onto the plane. Barnsby and crew successfully "escape" Termoli, only to be shot down by Luftwaffe night fighters. Only Barnsby, Mallory, Miller, Weaver, and Force 10 member Doug Reynolds (Angus MacInnes) manage to escape the crippled plane.
On the ground, the survivors run across a group they believe to be their allies, Communist Yugoslav Partisans, led by Captain Drazak (Richard Kiel). Force 10's rescuers are soon revealed to be collaborationist Chetniks (nationalist Serb guerrillas) under German control. The Germans take the team into custody but do not know their mission, so Mallory and Barnsby tell the commander, Major Schroeder (Michael Byrne), that they are criminals fleeing Allied authorities. To keep Schroeder from opening Miller's suitcase, which contains his special explosives, Mallory tells him that it contains a new wonder drug called "penicillin" which will spoil if exposed to air. By the next morning, the prisoners are told that Schroeder has opened the case, finding it full of firewood. Surprised, Mallory and Barnsby improvise an excuse, "admitting" they buried the samples. Schroeder sends them to retrieve them, under the guard of his concubine Maritza (Barbara Bach) and three of his soldiers. Miller, Weaver, and Reynolds are left in a cell in camp.
Far from camp, Maritza kills the Germans, revealing herself to be a partisan spy who hid the explosives before Schroeder opened the case. She directs Mallory and Barnsby towards the partisan camp under the command of her father, Major Petrovich (Alan Badel). Mallory and Barnsby escape, ambushing and killing two of Drazak's Chetniks, who are bandaged to hide burns from flamethrowers. Eventually, the two meet a patrol of real Yugoslav Partisans led by a man Mallory recognises as his target â Captain Lescovar alias Nicolai. While Mallory assumes that Lescovar has recognised him, too, he and Barnsby are nevertheless taken to the partisan camp, which lies near a wide river and a large hydroelectric dam.
Skeptical, Major Petrovich dismisses Mallory's story about Lescovar being Nicolai, assuring Mallory they executed the real Nicolai months earlier. He also reproves the men for killing the bandaged Chetniks, who were partisan infiltrators and his only link to his daughter Maritza. Petrovich's principal worry is an impending assault by the Wehrmacht: The Germans are marshaling their forces nearby, and only a ravine separates as many as three German divisions from the outnumbered partisans. Only a single concrete arch bridge links each side of the ravine, but the partisans have been unable to destroy it. Barnsby reveals that the bridge is, in fact, Force 10's target. Knowing Miller's expertise in demolitions, Mallory convinces a reluctant Petrovich to help mount a rescue mission, using Lescovar and the Partisan Marko (Petar Buntic).
The four re-enter the camp at night, with Mallory and Barnsby posing as captives, and Lescovar and Marko disguised as the bandaged men. Before they complete their mission, Drazak arrives with the bodies of the two real bandaged men. Since Maritza had always been seen with them, and realizing that she must have helped Miller and Mallory escape, Drazak attacks and beats her. Meanwhile, a gun battle breaks out in the cell block. Major Schroeder and Reynolds are killed; but Mallory, Barnsby, Miller, Weaver, Lescovar and Marko escape in a truck with a badly beaten Maritza and the recovered explosives.
Having made it to the Partisans, explosives expert Miller reveals that the bridge really is impregnable. Mallory hits upon the idea of destroying the upstream dam they saw earlier, with the sudden onrush of several millions of gallons of water being enough to destroy the bridge. A night-time air drop is arranged to replace Force 10's lost supplies, but Lescovar sabotages the drop by calling in German planes. Maritza spots Lescovar betraying the partisans, but he kills her before she can warn the others, and German planes bomb the illuminated drop zone.
Petrovich, angered by the botched air drop, orders the men to be sent to Marshal Josip Broz Tito's headquarters for transport back to Italy. The team decides to infiltrate the German marshaling yards at Mostar to steal explosives, with Lescovar and Marko volunteering to accompany them. Lescovar again betrays them, alerting a German sergeant to their presence, identifying himself as an Abwehr officer. Marko overhears the plan and sacrifices himself to save the others, who escape with Lescovar aboard a train leaving for Sarajevo. Lescovar tries to make up a hasty cover story, but cannot fool Mallory or Barnsby, and finally is forced to admit that he really is Nicolai. Barnsby kills him, and asks Mallory to return the favour by helping him accomplish Force 10's mission.
Jumping the train near the dam, the team splits up: Miller and Weaver set off diversionary explosives while Mallory and Barnsby sneak into the dam. Weaver runs into Captain Drazak in the woods and kills him in a knife fight. Mallory and Barnsby set their charges within the dam but realise that they are out of time. With the German assault only minutes away, they are forced to set a short fuse, leaving them no time to escape. Mallory and Barnsby are caught in the explosion, yet nevertheless survive. At first, the dam appears undamaged; but with its fragile structural integrity disrupted, the dam wall collapses, releasing millions of tons of water in a wave that topples the bridge. The German assault is thwarted, saving Petrovich and the Partisans.
Mallory and Barnsby rejoin Miller and an injured Weaver, but the jubilation is short-lived. Mallory reminds the men that they are now trapped on the wrong side of the river, with no supplies and no way of contacting or reaching the partisans. As the credits roll, the men begin a strenuous journey back to friendly lines.

Question: What have the partisans been unable to destroy?
Answer: concrete arch bridge