I would like to add a little more to the discussion around Foch as Generalissimo. It became apparent after March 21st that an overall commander was needed. The British had extended their line to the Oise River with Fifth Army. This freed up French divisions, which Pètain then held in reserve. Given that the British had fed more troops into the line, their reserves were correspondingly less. The German preparations for Operation Michael were very good. Much is made of the secrecy with which artillery and then infantry were put in place for the attack. But these preparations were not completely unknown to the Entente. A really interesting aspect of the German planning, which Ludendorff drew attention to in his memoirs, was the explicit goal of deceiving the Entente with preparations for attack along much of the rest of the Western Front. Thus, Haig was concerned about an attack near Ypres, whilst Pètain was concerned about an attack to the east of St Quentin. It is interesting to pause and reflect on these perspectives. Essentially, these views portray where the respective C-in-Cs perceived the main strategically important areas were, ie the areas that, if taken by the enemy, would have the greatest adverse effect on the war. Thus, the area around St Quentin was deemed relatively unimportant and was not defended as strongly. The weakness was detected by the Germans and led to this area becoming the target for Operation Michael. It exposes the problem with the German approach of attacking the weakest point - the point is weak for a reason.
St Quentin was only relatively unimportant. Plans were in place to support the defenders. The British had some reserves behind Fifth Army. The agreement to take over the line came with an understanding that Pètain would send some of the French reserves north to help Fifth Army if necessary. In the event, the German attack proved much more powerful and more successful that Haig had expected. Fifth Army had to fall back quickly to prevent it being surrounded. It wasn't just a case of feeding in a few reserves here and there. The further back Fifth Army withdrew, the wider was the potential gap between it and the French army. British cavalry helped plug this gap but the French reinforcements played a magnificent role in stabilizing this side of the German re-entrant. Meanwhile, Haig continued to worry about the north around Ypres, and Nivelle contined to worry about an attack in the Champagne region. Both wanted to retain reserves for these strategically more important areas, but, meanwhile, the German bulge continued to increase.
The British right flank, ie the remnants of Fifth Army, had to bend back, pivoting on the right flank of the British Third Army. This was the standard response to a threat against a flank - the German bulge was a bell-shape but the British would have focused most on that side of the bell that threatened them most directly. As the size of the Fifth Army shrank, and as the length of flank increased, more British reserves had to be sent south. Now, it was getting really serious. Pètain was focusing on the French perspective, doing just enough to protect their left flank but making sure that the possible attack in the Champagne region could be defended. This situation illustrates why the military has a hierarchical command structure with someone at the apex. If Haig's post did not exist, then the British Fifth Army might have only worried about its situation, and Third Army would have concentrated on its situation, with disasterous consequences. Someone had to draw things together.
It is very important to note that, contrary to what some people believed and wrote, Pètain responded quickly to the threat posed by March 21st.
Leaving aside what actually happened for a moment, I would expect that a Supreme Commander would have had:
1. The power to set overall strategy, in co-operation with the respective political authorities, and in consultation with the respective C-in-Cs.
2. Control over a small number of reserves that might be needed to support either the French or the British at the point of overlap (I don't just mean geographic overlap here). By 'control', I mean the ability to decide where the reserves should be allocated. Once the decision was made, the reserves would then fall under the direct control of the military commander to whom they were assigned.
3. The responsibility for ensuring that both C-in-Cs understood what was needed to achieve the broader strategic goals
4. The capacity to represent the needs of the C-in-Cs, particularly for the resources needed to fulfill the broad strategic goals
I would not expect the Supreme Commander to dictate precisely how the C-in-Cs then went about their jobs.
These views are based on my current limited understanding of how the chain of command worked, in general (if you pardon the pun), at that time. Please add to or amend the list.
So what actually happened? The post of Supreme Commander was ratified at the Doullens Conference. This took place on March 26 1918, only 5 days after the launch of Operation Michael. General Foch was appointed to the post. There are many different accounts of how the decision was made. These are interesting but not relevant to this discussion IMHO. It is important to note, however, that Foch had provided a memorandum to Clemenceau before the meeting. Doughty picks up the story:
'a memorandum in which [Foch] emphasized the importance of common Franco-British action and called for the appointment of an "agent" to give orders to allied forces and ensure that they were executed. In this memorandum he described the area south of Péronne, where Pétain and Fayolle had struggled to shore up the British Fifth Army..., as "easy to defend" and the area to its north as requiring substantial reinforcements by the French and British [he was a wiley fox
]. Though Clemenceau bristled when he read Foch's self-serving memorandum, Foch's optimism proved far more attractive to the grizzled politician than Pétain's pessimism.'