This was Frankie Pentangelli`s response to Michael as he was told about the assassination attempt at Lake Tahoe.

It`s always made me wonder why he said this when suggesting revenge on Hyman Roth.

Would i be correct in assuming that there are times when the families are weakened or low in "soldiers"?.

Why did Frankie need to mention "While we got the muscle"?.

I had always kinda assumed they always had the muscle.

Perhaps he was referring not to the Corleone's muscle, per se, but the Corleone's muscle in comparison to the soldiers that could be produced by Roth, the Rosato Brothers, etc.
In other words, let's hit them now while we have the upper hand, before they recruit more men to come after us with- a pre-emptive strike, if you will.

Sammy The Fish

I agree, Sammy. Frankie wanted to convey a sense of urgency to persuade Michael to give the order to kill the Rosatos and Roth.

Kind of like those home shopping shows that have a clock ticking down on the sale seemingly to encourage the viewer to buy and buy quickly.

This scene stays with me because it reveals the opposite natures of both men. Frankie is visceral and impetuous, speaking in concrete metaphors (muscle) with emotion. Earlier in the discussion he acknowledges something to the effect that he doesn't have Michael's brain for business (again a concrete metaphor for an abstract concept) and this dispute with Roth/Rosatos was a "street" thing (again concrete).

Michael is cerebral and calculating. Raising his voice to Pentangeli when they first met in the scene is not a spontaneous act of excitement, but rather a deliberate, premeditated act to put Pentangeli a bit on edge.

When Michael responds to Pentangeli's comment about getting them while we have the muscle, he resorts to a much softer tone and talks about the room, the desk where his father would be, the friends occupying the house to lead up to explaining the concept of keeping your enemies close. Michael's little story of his father in the house was a way of explaining this concept in a way that Pentangeli could understand and appreciate.

Good points by all!
One way to look at Frankie's statement is to look at it in the context of an earlier statement: "Michael, I...I don't have your head for the...uh..big deals..." A guess is that Frankie was being somewhat scornfull--as if "big deals" were a weak substitute for real action. So, when he said, " WHILE we got the muscle [emphasis added], he may have been implying that Michael was pissing away the family's muscle in favor of some weak "big deals."

I think what Frankie was talking about was that the Rosatos were moving in on his territory, and Michael told him to acquiesce to it. From Franlkie's point of view the success of the Rosatos would weaken him, and he would have less "muscle." Thus when he heard that Michael not only wanted Frankie to back off, and that it was Roth who tried to assassinate Michael, his reaction was to go after all of them now ....before the Rosato's moved in, and while the Corleone family, as frankie saw it "still had the muscle."
When Mike says no, then Frankie is perplexed. He sees Michael as giving up street territory in New York to Frankie's detriment, and he also sees that Michael is making some kind of partnership with Roth in Cuba. At that point he says he doesnt have Michael's head for "big deals." Michael halfway answers the question by saying something about keeping your enemies closer, but he stops short of giving any details.