It's a not-entirely fictional story about the Ango-American landings at Anzio on Italy's west coast. It's diluted because the story behind those landings is far more interesting than what we see on the screen. Names, personalities, and motives are changed around so that hardly any echo of the real characters remains, although we get a lot of information about characters created in the screenplay.
Basically, Robert Ryan plays General Mark Clark who was in charge of the operation and was in overall command of the Fifth Army. He was an interesting guy for a general -- tall, vain, brave, half-Jewish, a large-featured face like the mask of Tragedy, carrying around a sidearm as a prop. Arthur Kennedy plays General John P. Lewis (modeled after Gen. Lucas), in charge of the landings themselves. Mitchum accuses him of being "timid" (three times) and in a way he was, although it wasn't entirely his fault. Arthur Franz has a small role as General Lucian K. Truscott, the junior general in command of the Third Division (Audie Murphy's division). All the names have been changed to protect the guilty.
Here, basically, is how it worked. The Allies of half a dozen nationalities were being slaughtered throwing themselves against the German Gustav line, which ran across the Italian boot from sea to sea, commanded by the unconquerable Monte Cassino. The Anzio landings were designed to catch the enemy by surprise from behind and relieve pressure at the Gustav line. Mark Clark (who saw to it that any reference to the Fifth Army in the press appeared as a reference to "Mark Clark's Fifth Army") had supervised similar earlier landings at Salerno. They were successful, but just barely. The landings at Anzio were handed over to Lucas, whose orders included a drive inland, if possible, to the Alban Hills which commanded a perfect view of the beachhead and the main highway to Rome. Lucas had just troops enough to dig in and consolidate or to race pell mell to the Alban Hills, but not enough troops to do both. Lucas was advised by Clark, "Don't stick your neck out like I did at Salerno." (The line is directly quoted in the movie, but is given to Robert Ryan's fictional general.) So Lucas didn't stick his neck out. He went inland seven miles, stopped short of the Alban Hills, and dug in. Clark, who was on the beach, agreed with the decision.
And Lucas wasn't the cocksure but mistaken strategist played by Arthur Kennedy. The real General Lucas kept a diary and it's full of gloomy forebodings. The Germans, under Kesselring and Mackensen, were caught unprepared. Nothing stood between the allies and the Alban Hills, or Rome for that matter. But Lucas did nothing, and for good reasons. He didn't have the resources to take Rome and hold it. Except for the probe by Rangers, as shown on screen, and others by British troops, everyone dug in and waited for the German reinforcements to deploy, which happened apace. Kesselring was a very efficient tactician and had plenty of time to bring in troops in the stalled Allied beach head.
Instead of Anzio rescuing the troops at the Gustav line, the situation was turned around. In the end, some 24,000 American and 9,000 British casualties were evacuated from the beachhead. Clark fired Lucas and gave command to Truscott. When the German resistance finally collapsed, General Clark had an opportunity to drive eastward across the Italian boot and cut off the German troops to the south. He chose instead to forget about capturing the German army and to zip his own troops north along the highway to Rome so that he could "conquer" the open city. You know -- like Julius Caesar? The German army promptly withdrew north to their next massive defense line, where the Allied advance stalled again.
I leave it to the viewer to decide which story is more engaging, the historical one or the plot we see on the screen, which is mostly the story of seven survivors of the Ranger patrol who try to make their way back to Anzio, a story we've seen many times before. I wish I could at least say that the story presented on screen is well done but the fact is that it's not. This is one of Mitchum's lazier performances. Sometimes he sounds positively drunk. No one else stands out, including Peter Falk, who has the reactivity of a noble gas. Arther Kennedy is the smarmy General Whatever-his-name-is. And some other posters are absolutely right about the score. Whew! A simple-minded would-be catchy love song doesn't turn into a martial theme just because you throw some snare drums behind it and play it as a march.
What a missed opportunity.