All right, got distracted by some space threads but I'm caught up again!
I am pretty amazed that the Massaliotes were able to stop and ultimately defeat Scipio's Roman army. I've been thinking of this like Allies Versus Axis in WWII, with Rome and Carthage as the Axis. OTL from 1939 to 1942, the Germans and the Japanese were able to keep the British, Soviets and Americans on the back foot, having indeed wiped out France completely and shattering Dutch power in Indonesia and overrunning everything east of Burma and north of Australia no matter who held it. In mid-1942 it looked like the Axis might win. And if you asked Hitler or Tojo, they'd tell you that the key was boldness and decisiveness and preparation for a winning campaign. If their enemies had been like them they'd have been more suspicious and better prepared, but being liberal democracies they had neither the coordination nor the willpower for a fight they were losing. Bold strokes win all! It was only after some years of enduring these bold strokes and unaccountably not quitting that the British combined their own gradual transformation into a fighting machine with gaining the USA (pretty far behind in that same process) and Soviet Union (forced to accelerate it while being mauled) and the lines firmed up to define maximum Axis advances. And after that, the great miscalculation of all the myopic Axis leaders, that their foes could and would develop resolution when they needed, could muster up some boldness of their own, became masters of decisiveness as the need for focus took hold, and then enjoyed to the full their inherent but hitherto discounted advantages of deep material and broad personnel resources with superior logistics and made the ultimate and total defeat of the Axis inevitable.
Here too, I figure the Romans discount the sheer breadth and depth of Massaliote possessions and people giving it allegiance, because they discount its leadership as decadent, pleasure seeking, and while they know the tagmas are forces to be reckoned with, they underestimate both the resolve of the League to use them if their own heartlands are threatened and the ability of the League to muster competent forces.
So like Hitler planning his blitzkrieg strokes, the Romans would not strike at Massalia until they judged they had sufficient force at hand to overwhelm everything the League was known to have in the region, and anticipating that more tagmas would be sent from Tolosia and possibly even Iberia to reinforce the defense of Massalia, they too would prepare slower but larger forces in reserve to aid Scipio. If those forces were not to hand, I judge, they would not launch the attack half-cocked.
The goal I figured was to force the League defenders back into a siege of Massalia, and therefore after Scipio's quick transalpine strike in the northeast, which was suitable to engage the light forces present, more legions should have marched over that and other passes, so that as League forces mustered to turn on Scipio, they find yet more Romans coming at them from other directions; Scipio would feint at the city but sacrifice that apparent surge as a cover for regrouping with the reinforcements, making a really huge army--as huge as it took to hold the tagmas and auxiliaries the League could muster immediately at bay and force them into the siege. At that point too the Romans, having studied the problem, would have solutions to to take and sack the city.
And at that point they'd expect the wind to fall out of League sails, and the remnants to come to terms. Up until the fall of Massalia, I'd expect the Roman plan to go swimmingly; only after that when the League unaccountably refused to lie down and realize it was dead would things start to come unglued as forces calibrated to achieve the goal of destroying the city of Massalia find themselves sinking in a quicksand of millions of unexpected tiny cuts.
I expected it to be epic, terribly tragic for both sides but especially the League, as WWII was in the east--but like the Soviets, the League draws a line, turns the tide and then marches unstoppably toward Rome. Because these are classical times and because the League would be decimated, they might accept a truce once the Romans are back over the Alps, but from that day hence League and Rome are locked in an existential struggle that nearly bleeds the life out of both, but also from that first truce, indeed the first reversal of Roman advance, Rome's days are numbered as the League now will not rest until Rome is eliminated as a threat. Massalia gets rebuilt of course.
Which is one reason I figured the endgame between League and Rome would take not months, or even a couple years, but decades to complete.
What happened instead shows that either Roman intelligence of League capabilities in the region under informed Roman leaders horribly, or that Romans were much too overconfident the mere sight of a legion would paralyze Greco-Gaulish will and hence defenses. They appear to have underestimated not only the subtle factors but gross ones too, like how fast a tagma could be mustered and then marched across country.
Since they have been turned away from sacking the queen city, now League culture is in less immediate danger and its material forces are stronger, but by that same token they are perhaps not sufficiently impressed with the Roman threat.
In short it isn't like WWII at all, where the two sides formed mutual existential threats to the other and neither would make truce with the other but insisted on wiping it out completely. It may be a lot more like other periods in military history, where two sides fight and bleed one another but then settle for a truce leaving both still standing.
But I forget there are yet more lessons and more ways in which the situation differs from modern ones. In modern nations, there is no way that an amphibious descent on a port followed by direct advance on an enemy capital could work; it would be a salient quickly cut off, and there is no way such an expedition could have the punch necessary to take the capital. But in these ancient times, populations are lower, great empires are generally cobbled together with relatively small forces intimidating populations into client status, and such a move as Philopoemen's gamble can have dramatic effect, if it pays off. This is what the Romans figured they could do to Massalia after all, and that it would demoralize the League.
Now as I understand it, OTL Rome was rarely sacked, but happened a few times, and the Romans did not curl up and die nor lose all control of their clients--in the days of the Celtic raid to be sure, the Republic was on a different basis, much more local and I gather somewhat more equal in their treating of the more or less recently brought in line Latin League. And I believe some clients did take the opportunity to rebel. Certainly the next time Rome was threatened with destruction, with Hannibal scouring the peninsula OTL, many cities did rebel (not Massalia, but many of her neighbors in Liguria, Genua being the exception). Yet again the Romans did rally, so it is not so clear that if one cuts off the head of the snake it dies. (It is probably true that if an army spends years fighting its way up to the head of the snake, slicing its metaphorical body to little dissected bits, then destroying the "head" would be finally decisive. But this is just the sort of quagmire Phiopoemen hoped to avoid floundering in.
We remain in suspense, as I write, whether his gamble pays off or goes bust--even if it does and Rome is sacked, I am no more sure that is the end of Rome than I would be if Massalia fell to them it would end the League. Scipio has lost his army but he's still out there, coming back from the northwest; surely he has mustered what is left of his original army and augmented it with people he picks up in the Po valley, and will lead these, gathering others, to the relief of his city, or to avenge it should he get there too late. Other Roman leaders scattered over Italia are probably mustering forces (knowing they either need to sacrifice Rome's southern conquests yet again, or leave some trusty men behind to hold them down) to the relief of Rome too. If the city falls, they might converge to take revenge, or sit back and wait for the jackals to go away so they can begin rebuilding. Such a stroke will surely weaken Rome and delay her strength, but it can only harden Roman resolve to avenge it someday.
But perhaps with the fall of the city, enough subject peoples will rise up to absorb and scatter the rallying legions, and perhaps we are about to see the final fall of Rome and the abortion of Latin domination of European culture.
I imagine the likely thing is that Rome will be around for a few more innings. On the other hand they have an enemy as never before in the League.