In the film, he makes that arrangement in order to bring Michael back safely. It seems unlikely that he would risk his son's life on a bit of cheap wordplay. The other families surely wouldn't respect the truce in that instance anyway.
He also swears that he won't be the one to break it "on the blood of his grandchildren". Shortly after, we see him plotting with Michael against the other Families (partly because they know Barzini has a mole in theirs), and at the end, Michael has all the other heads assassinated... and come the third movie, Vito's grandaughter by Michael is killed by a bullet meant for his son.
Michael was widely regarded to be a noncombatant by the different Mafia families, despite being the son of Vito Corleone, due mainly to the fact that he stayed out of the Family Business, kept his nose clean, and was a certifiable war hero. He might as well not be part of the family as far as they are concerned, when there are far more relevant targets such as Sonny (or Vito himself) whose deaths would actually affect how the family was run. Once Michael gets drawn in, he is shown to be very ruthless, and very cold. When asked what he would do after killing Solozzo and McCluskey, he replied that he would sit down and finish his meal. When his godson is being baptized, he has his men do a mass-killing of the rival family heads and Moe Greene. In the second movie, he is told that he pissed off a lot of people when he did that, as it wasn't how things are done, and it is implied that this bought him a short term advantage at the cost of long-term problems. Why does he take this approach? Because he doesn't have any direct background in the Mafia. He is a soldier, who was trained to deal with his enemies far more directly than what is considered appropriate in the American Mafia. If someone is an enemy and is dangerous to you, you kill them before they can do the same to you... if you are talking about two armies on the battlefield. He is Wrong Genre Savvy, but dangerously so.
Michael may not have been a mobster originally, but as stated above he was a soldier. In the mobster world, you don't touch officers. But in the world Michael was used to, the military world, you kill "officers" if they're not on your side. An opposing officer's stripes would not deter him on the battlefield. Michael could look at McClusky as something other than untouchable because of his background. McClusky was just another enemy soldier to Michael, who'd made the mistake of breaking his jaw.
The opening scene of The Godfather gives us Amerigo Bonasera asking Don Vito to kill (implied) the men who ravaged his daughter as vengeance, describing how "she will never be beautiful again." This scene is mirrored later in the film after Sonny's death, where we find Don Vito asking Bonasera not for vengeance, but simply for him to use his skills as an undertaker to make Vito's son beautiful again.
Originally, Clemenza was supposed to reappear in Part II, but actor Richard Castellano had a disagreement with the filmmakers, so his role was filled by a new character, Frank Pentangeli. In the film, Pentangeli survives an assassination attempt he believes came from Michael Corleone, and becomes a witness against him in the senate hearings investigating organized crime. Corleone brings Pentangeli's brother from Sicily to the hearings, which causes him to recant his testimony. Later, Tom Hagen gives Pentangeli Michael's offer to spare his family if he commits suicide. This wouldn't have worked if they had had Clemenza. In the scenes set in the past, they show young Clemenza (played by Bruno Kirby) crossing himself before eating. It shows that though he might be a murderous thug, he's still a good Catholic, and a good Catholic wouldn't commit suicide!
In Part I, Vito says to the other dons, "I swear, on the souls of my grandchildren, that I will not be the one to break the peace we've made here today." Technically, he kept his promise, but in Part III, his granddaughter Mary, an innocent civilian, is killed by a bullet meant for Michael.
The ending to Part III. Before the passing of Mario Puzo, he and Francis had plans to make the next Godfather as Vincent's time as the Don, and he would have gone into the drug trade. It would have ended with his death sometime in the mid-90s, after being hunted down and killed in a police shootout. Taken this account, Michael's death in '97 makes it all the more tragic since the Corleone name was by that point a worthless name because of the damage Vincent had done. Michael Corleone would have truly died broken and alone.
Technically, according to a "Corleone family tree" found in the Coppola Restoration compilation as an extra, Vincent gets gunned down in 2005 after soiling the Corleone name by getting into the drug business.
Carmela (Vito's wife) is actually the bad apple in the marriage: all the Corleone boys have terrible defects not present in Vito: Sonny's violence, Fredo's vices and Michael's cold heart. Since all this didn't come from Vito, then it had to be from their mother's side who doesn't appear that much to be noticed as an actual evil person. In a way, she might be the Bigger Bad or even The Man Behind the Man since in the part II flashbacks she's the one who introduced her husband to the first of his "godsons" asking favors.
When I was playing The Godfather game, I wanted the Mob Wars to be fully playable, with running battles in the streets. Thus, I was disappointed when they boiled down to either taking out an enemy business or simply running to a FBI agent and paying some dues, with no bunches of gangsters popping out of the woodwork to rumble with. Some more playtime and deaths later, I came to a realization: With the game's near-Nintendo Hardness because of its pseudorealistic damage model, having to fend out randomly popping out mobsters may be realistic but definitely more frustrating than fun. Since the game may already try one's patience normally... — Gentlemens Dame 883