The following uses the translation of
Titus Livius (Livy), The History of Rome, Book 21 Benjamin Oliver Foster
Livy. Boooks XXI-XXII With An English Translation. Cambridge. Cambridge, Mass., Harvard University Press; London, William Heinemann, Ltd. 1929.
[19.1] This straightforward demand and declaration of war seemed more in keeping with the dignity of the Roman People than to bandy words regarding the rights involved in treaties, especially at that moment, when Saguntum had been destroyed.
The whole area was disputed and Carthage and Rome made treaties detailing the spheres of influence. These treaties were apparently less than ideally phrased to perfection and unambiguity. In short: both sides argued for interpreting these agreements as maximally in their respective favour, leading to an overlap of zones.
Livy argues that after the destruction of Saguntum Romans were even less inclined to argue over minutiae of any treaties. They felt that Saguntum was their ally and Carthage had no right to interfere. While that may have been settled with negotiations beforehand even by or with disgruntled Romans, the fait accompli the Carthaginians presented them provoked the anger of Rome. And Romans preferred to fight such things out anyway.
Note: against other historians like Fabius Pictor, but following Polybios, the emmissaries to Carthage before the fall of Saguntum seem to be a polybian invention, constructed to indisputably paint Hannibal and Carthage as 'the baddies knowing what hey were getting into.' Other historians noticed discrepancies in the traditional account as well, but Livy analyses them, that is he (dis-)'solves' these in a pro-Roman fashion.
Note: It is unclear whether Saguntum really called for Roman help before Hannibal laid siege to it. But it is clear from Livius that he paints Hannibal as uninterested in treaties and negotiations, but in favour of letting arms speak. Following Polybios in this whole affair in this regard seems to build a castle onto sandy grounds.
 Though for that matter, had it been proper to debate the question, what comparison could there be between Hasdrubal's treaty and the earlier treaty of Lutatius, which was altered?
Livy goes on to highlight what a mess these treaties apparently were. Content for both sides at the time of signing, but not clear enough to prevent later arguments from the text alone. Two treaties signed, both with ambiguities and unclear provisions, unclear whether they should be mutually amending or one superseding the other.
Note that both were drawn up in haste to prevent all out fighting, as a face saving measure for both sides. ("Peace in our time") It is unclear whether any treaty really needed ratification by either the senate or the gerusia to be 'in force' or binding.
 For in the treaty of Lutatius it had been expressly added that it should be valid only if the people ratified it; but in Hasdrubal's treaty no such proviso had been made, and by the silence of so many years the treaty had during his lifetime been so sanctioned that even on its author's death no slightest change was made in it.
Note: The treaty of Lutatius ended the First Punic War and was formulated in two distinct steps: a preliminary peace treaty agreed upon by Hamilkar Barkas
and last years consul Gaius Lutatius Catulus. That treaty the Roman senate didn't like and so they sent 10 more men to negotiate to Carthage.
Quintus Lutatius Cerco, brother to Gaius Lutatius Catulus, led that group to renegotiate the preliminary contract, that contained the following conditions:
- Evacuation of Sicily
- Waiver of military action against Syracuse
- free/unpaid release of the Carthaginian and Roman prisoners of war
- Reparation payments of 2200 Euboeian talents within twenty years
to the legally binding Lutatius contract, which contained the following conditions:
- Evacuation of Sicily and all islands between Italy and Sicily
- Waiver of influence of all alliance partners in the sphere of influence of the contractual partner
- free release of the Carthaginian and Roman prisoners of war
- Reparation payments of 2200 Euboeian talents within ten years, as well as 1000 Euboeian talents immediately
Hasdrubal's treaty on the other side is more commonly known as the Ebro treaty. That was seen as demarcation of the actual zones of influence on the Spanish peninsula. In it is was said that Carthaginians were not allowed to cross the river Iber. Unfortunately it is unclear which river was meant with Iber and is a matter of contention to this day whether it contained any specific mention over Saguntum. It can be argued for either side that they either stood their ground or sought a pretext for war. It seems likely that Hasdrubal wouldn't have had reason to bind himself one-sidedly and on the hand the Romans seem to have called more than one river Iberus. Consequently it remains unknown whether Saguntum was north of the river or south of it.
 And yet, even if the earlier treaty were adhered to, the Saguntines had been sufficiently protected by the provision made concerning the allies of both the parties; for there had been no specification of “those who were then allies,” nor exception of “such as might [p. 55]afterwards be received.”
Meaning that suboptimally phrased treaties once signed over 'zones' are irrelevant once times change and allies join the Roman side. In other words, old treaties are now like water under a bridge over a river nobody can pinpoint anyway.
Rome initially negated then renegotiated the first treaty. Now the Carthaginians say that Roman complaints over the second treaty are nought as that never came into effect (and indeed did not exactly in the same way like the first one.) But for Livy/the Romans that is again irrelevant as the Roman side gives more weight to the bilateral relationship between them and Saguntum. As if any treaty contained the clause that "specifics to be drawn up in later treaties", instead of interpreting this as "this is the general agreement, breaking smaller ones should they come into conflict of content".
 And since they were permitted to take new allies, who would think it fair either that they should admit no one, however deserving, to their friendship, or that, having once taken people under their protection, they should not defend them —provided only that allies of the Carthaginians should not be tempted to desert them nor be made welcome if they left them voluntarily?
As spheres of influence only talk over the rights and duties of the signees, but Saguntians as non-signees remaining free, in Livy's account, they had the right to defect to the Romans, and once recognised by the Romans as 'friends', Rome then 'had the duty' to defend them or to cry revenge in the name of freedom and democracy. Actually, delete the last three words in the preceding sentence. They are always lies.
Livy doesn't criticise individual Carthaginians, but them. While this passage does provide for a possible Roman rationale in argumentation, it is a one-side report that justifies a war that was won. A seemingly 'just war' – bellum iustum – is much more popular. Since the actual causes of the war were disputed in Livy's time. But the exact passage above is designed to emphasise that Rome considered the legal position 'perfectly clear'; when it really seems to have been just 'no longer changeable'.
– Pedro Barceló: "Rom und Hispanien vor Ausbruch des 2. Punischen Krieges", Hermes 124, 1996, p45–57.
– Peter Bender: "Rom, Karthago und die Kelten", Klio 79, 1997, p87–106.
– Klaus Bringmann: "Der Ebrovertrag, Sagunt und der Weg in den Zweiten Punischen Krieg", Klio 83, 2001, p369–376.
– Adrian Goldsworthy: "The Punic Wars", Cassel, 2000.
– Fritz Moritz Heichelheim: "New Evidence on the Ebro Treaty", Historia 3, 1954, p211–219.
– Friederike Heubner: "Hannibal und Sagunt bei Livius" , Klio; Jan 1, 1991; 73, p70.
– Krešimir Matijević: "Der Ebrovertrag und die Verantwortlichkeit für den 2. Punischen Krieg", in: Gymnasium 122, 2015, p 435–456.
– Ralf Urban: "Roms Gallierkrieg 225–222 v. Chr. und der Ebrovertrag", in: Klaus Geus & Klaus Zimmermann (Eds.): "Punica – Libyca – Ptolemaica. Festschrift für Werner Huß", Leuven 2001, p. 277–288.
– John Serrati: "Neptune’s Altars. The Treaties between Rome and Carthaga (509-226 BC)", in: The Classical Quarterly 56, 2006, p. 113–134.
– Klaus Zimmermann: "Rom und Karthago", Darmstadt 22009.