Were there serious concerns that FDR (or Churchill for that matter) could have become targets if enemy intelligence got wind of their travels?
Not all that risky really. The Germans had no reliable spy network to let them know about any arrangements and the leaders often travelled under false names to confuse things (Churchill was Col Kent I think).
Even though FDR (and later Truman) had the greatest distance to travel, it was always Stalin who repeatedly renegotiated the locations and dates of these conferences to insure his own safety. In fairness, Stalin didn’t have capital warships at his disposal nor was air travel over Eastern Europe a very safe option at the time.
The Soviets said there was a plot by German intelligence to assassinate the Big Three in Tehran. They claimed that they found out about the plot, called Operation Long Jump, and caused it to be called off by surveilling the advance team. After the war the Germans denied it and there is no consensus as to whether it actually happened.
Unlike the European leaders, the U.S. Chief Executive had the obligation to physically sign or veto legislation in a timely fashion. According to Churchill, this was a major reason — or at least the explanation given to Stalin — for FDR’s reluctance to travel as far as Tehran. Bills would need to be flown to Tehran for signature, then flown back.
But it made good movie stuff. With Alain Delon! 
The Soviets also had Yalta bugged up to the nines so they could listen to what their frenemies were discussing in private. Churchill, at least, was aware of this - don’t know if FDR was.
IIRC there was a large danger as Roosevelt went to Casablanca except when the spy informed Germany that he would be in Casablanca they translated as the White House and so did not make an attempt on him. But I’ve always thought that story was an urban legend.
LOL.
ring-ring “Hallo, Oberkommando der Wehrmacht. Heil Hitler!”
“Heil Hitler! This is Agent X42! I have urgent information: President Roosevelt is in White House! I repeat: President Roosevelt is in White House!”
“Oh, uh, very good, Agent X42.” hangs up phone, rolls eyes, makes jacking-off motion
“Who was that?”
“Oh, just Agent X42 again.”
“What, again?!?”
“Ja, third call this morning.”
In Roosevelt’s case the strain on his health was probably a concern as well. He would have been at higher risk for problems like DVT considering he was wheelchair bound combined with the length of the trip.
He was a very ill man at Yalta, and so was Harry Hopkins by that point. If FDR had been well, I very seriously doubt that Stalin would have been able to pull off his European land grab.
I find that hard to believe. It seems most of the lines were drawn where the armies stopped, and I doubt anyone had an appetite to fight over the spoils to try to move the finish lines. In fact, Stalin graciously(?) allowed he other allies a share of Berlin.
Plus, IIRC, some of the more interesting take-overs happened in the next few years as Soviet-backed parties took over the newly liberated countries in coups disguised as elections, long after FDR bit the big one. Where the Soviet army did not make to, like Greece, the take-overs failed with western help.
I thought the armies stopped at agreed upon lines.
The US went past the agreed dividing line in Germany in some places. Later withdrawing. Since people didn’t trust Stalin plus interest in grabbing German scientists and tech led to rapid pushes to the line and beyond. The Allies were seriously worried, for example, about the Russians moving up the Jutland peninsula and taking over Denmark. So Monty raced to the Baltic to stop that. (Russia did grab a Danish island and held onto it for far longer than needed. Took a lot of protests for Denmark to get it back.) Russia also was in a part of N. Norway for a while but they withdrew since there wasn’t anything worth fighting for up there, esp. after the Germans did their scorched Earth thing.
As to the other countries it was grab what you could. Patton thought he could reach Prague before the Russians which, if true and he was allowed to try, might have changed things there quite a bit.
Austria was divided as the armies met. It’s one of those weird things that Stalin eventually agreed to pull out and Austria was united again.
There was also a big mess near the N. Adriatic as various factions fought over claims to Trieste and other regions. But one side was Tito, not Stalin.
Indeed. His blood pressure would soon become legendary.
