Both sides have too different a view on how to engage in war in Syria, which has led to the US cutting off ties and an inevitable backlash from Russia.
The US is angry, because there is evidence to suggest that Russian munitions have fallen on civilian areas and hospitals, and humanitarian access has not been granted by Damascus to besieged areas.
What exactly happened with the attacked humanitarian convoy is not completely clear, but Bellingcat has offered information as to why it was likely to be an airstrike, with Russian aircraft thought to be in the area at the time. (Bellingcat's website says its citizen journalists use "open source and social media investigation to investigate a variety of subjects" -- in this case, Russia's airstrikes on targets in Syria.) Russia may not realize it, but its immediate defensive stance, and the subsequent offering of its own multiple and conflicting theories as to what happened, raises suspicions that they may not be telling the truth, regardless of whether they are or not.
Russia is angry that the US has not fulfilled its side of the bargain of separating out terrorist groups from the "moderate" opposition. The delineation of territory controlled by ISIS, Jabhat al-Nusra and opposition groups was set as a priority, as per the US-Russia agreement that the Russian Ministry of Foreign Affairs decided to publish.
Russia is concerned that the US is purposefully refraining from doing this in order to maintain leverage over Syrian President Bashar al-Assad. Separating out al-Nusra is of particular difficulty, given that they are intermingled with opposition groups, so one wonders how the US envisaged the agreement being implemented. Why Russia repeatedly uses this to justify the bombing of civilian targets by the Syrian government and Russian air force, however, is unclear.
Due to the fact that trust has slipped to a renewed low, the US has decided to suspend talks with Russia on Syria. Russia has blamed the US for the agreement's failure.
Did Russia care much about the actual implementation of the ceasefire deal, agreed to last month? Or was it simply a diplomatic coup to be seen as a peace-broker with the US? It is likely that Russia was interested in a deal that at least began a move toward ending the conflict, but the approach and objectives of Russia and the US have been too different and, at times, mutually exclusive.
Russia has picked up on the West's hypocrisy in the latter's claims to a foreign policy based on the moral high ground in the Middle East and wishes to challenge it. To be fair to Russia, most would be skeptical about how much trust the US can place in the array of groups it considers to be the moderate opposition.
A key Russian objective in the conflict is also to demonstrate that it will not stand for what it sees as the West's obsessive tendency towards "democratization," i.e. the ousting of brutal leaders as a means by which to solve conflict in the Middle East. The Western mantra that "Assad must go" demonstrates that both sides have been talking past each other for most parts of the Syrian conflict. This means that Russia feels the need to support Assad and the Syrian government, at least in the short-term, but in doing so is maintaining the central sticking point to any political resolution.
So what does Russia actually want to happen?
This leaves both the US and Russia in awkward positions. Neither can truly and definitively "win" the conflict alone, but they also cannot seem to find a way to cooperate.
Given how frustrating Russia's reactions have been to simple issues of access for humanitarian aid and seemingly purposeful bombing of hospitals, one could argue that the ball now sits in Russia's court.
But there is little evidence -- as demonstrated by sanctions on Russia as a result of its actions in Ukraine and Moscow's tepid reaction to those sanctions -- that Russia is in the mood to concede any diplomatic ground to the US.
For other examples of Russia's current thinking, see Monday's announcement that it was suspending an arms reduction agreement due to the US's "unfriendly actions" toward Russia.
Russia wants to fight the conflict in Syria on its own terms, which are at odds with those of the US. With America walking away from the table, Russia will now be more intent than ever on not losing ground on any issue in Syria, including compromise on possible solutions. It will most likely be up to the US to open the door if talks on any solution are to restart.