In Part II, I never understood what the deal was with the military police that kill Michael's assassin just before he is about to smother Hyman Roth. How did they know he was in Roth's hospital room at that very moment?

(Does the scene just prior to that when Michael asks Fredo where he is going and he responds that he is going to get himself a "real drink" have something to do with this??
After Fredo walks away, there is a shot of Michael's face and he is visibly suspicious. Then, just moments later, Michael sees the military police marching through the ballroom, and Michael looks a bit worried and intrigued by them b/c obviously he knows that it has something to do with his arranged murdering of Roth...I was always confused by this; Did Fredo, knowing of Mike's plans for Roth, tip them off? (cause thats what this sequence of events kinda suggests.)By then why would he help roth?

Certainly, Fredo making a call is a possibility. As to why he would help Roth, the level of involvement between Roth and Fredo has been a subject of much debate on the boards lately.

Another possibility is that Johnny Ola's body has been discovered, and Roth's wife or some other associate realizes that Hyman is in danger and requests protection for him.

Of the two explanations offered by LW, I'm more and more favoring the scenario that when Fredo went out "to get me a real drink," he was tipping off the police. Go here to check out the discussion LW was referring to:

http://www.gangsterbb.net/threads/ubbthreads.php?ubb=showflat&Number=472494&page=2&fpart=1

The troop of helmeted soldiers who marched into the Presidental Palace during the party had no connection with the military detail that shot the bodyguard at Roth's bedside. The former were probably giving Batista an update on the deteriorating situation with the rebels, and were arranging his escape from Cuba.

My guess is that the police probably had a tail on Roth and they were notified immediately he was taken to the hospital.

I disagree. Theh deteriorating security situation on New Year's Eve prompted Batista to send a military escort to the hospital to retrieve Batista's good friend, Hyman Roth.

How did they know he was in the hosital?

The Lake Tahoe assassins told them.

He was the first guy with AARP.

Agreed. Michael tells Fredo "Hyman Roth wont see the New Year" Fredo's tips them off then Roth is saved just before the New Year. Plus as LW mentioned the sequence of events from "real drink" to "guards marching, Michael seeing" etc.