Also, how did kings get an accurate assessment of their own forces?

As an addition, I'd like to ask how prevalent espionage was back then, if it was effective and or accurate and were there accounts of, "double agents?"

I may not be able to give you a complete answer as I am not able to speak intelligently on the levy system to which you refer. Hopefully another qualified individual can go into more detail on this subject. However, I am familiar with how the Byzantine Roman Empire of the Late Antique, and Medieval period waged war and hope to shed some light on how they went about, and thought about sizing up their enemies during this period.

The Byzantine Romans were extremely conservative in their military exploits, and only risked direct engagement when odds were much in their favor. In this sense, it didn't matter how many troops their opponent could field in total, they would avoid superior forces, destroy smaller ones, use attrition and ambushes to wear down their opponents and find other regional players or rivals to attack at opportune times.

To quote the emperor Maurice (who reigned a little before your time period, but the point remains the same for later emperors and generals)

Warfare is like hunting. Wild animals are taken by scouting, by nets, by lying in wait, by stalking, by circling around, and by other such stratagems rather than by sheer force. In waging war we should proceed in the same way, whether the enemy be many or few. To try to simply overpower the enemy in the open, hand in hand and face to face, even though you may appear to win, is an enterprise which is very risky and can result in serious harm. Apart from extreme emergency, it is ridiculous to try to gain a victory which is so costly and bring only empty glory.

And another,

A wise commander will not engage the enemy in pitched battle unless a truly exceptional opportunity or advantage presents itself.

Later on, in the 10th century, the Emperor Nikephoros Phokas instructs on how to gather information to ascertain if open battle can be risked,

It is imperative first to find out the number of the enemy host and above all what kind of equipment they have, by means of spies, deserters, and prisoners. If the enemy force far outnumbers our own both in cavalry and infantry, avoid a general engagement or close combats and strive to injure the enemy with stratagems and ambushes. The time to seek general engagements with the enemy is when, with the help of God, the enemy has fled once, twice, or three time and are crippled with and fearful, while on the other hand our host is obviously confident and their thoughts of valor have been awakened. Avoid not only an enemy force of superior strength but also one of equal strength, until the might and power of God restore and fortify the oppressed hearts and souls of our host and their resolve with His might hand and power. And when this love and compassion of God is stirred by the intercession of the all-celebrated Mother of God, when She secures Her people's victory for the third time, from that moment on they need not flinch or recoil in fear, be the enemy twice the number of our host.

The religious language not withstanding it is important to note that Nikephoros will not engage an equal or larger enemy but avoid confrontation, ambushing and harassing, until the most opportune time. Only then will he attack, and if the enemy's morale and will is broken the numbers do not matter.

To further expand on these stratagems, we have another source by the soldier and general Nikephoros Ouranos, who also wrote in the 10th century.

The commander of the army must first make investigations through spies, prisoners, and deserters and find out the situation of each enemy area, of their villages and fortresses, as well as the size and nature of their cavalry forces. When he is well informated about all these matters, he must take into account the time of year, and he should make incursions and raids with due allowance for the season and the accessibility of the areas.

[while on the march] he should give order to the officers that each of them must detach one hundred or 150 of his light infantrymen and convert them into cavalrymen to accompany the cavalry force…. Similarly, forty or fifty of the kataphraktoi should be set apart, who will leave their heavy armor and that of their horses with the baggage train and head out with the other light horsemen. When the army approaches the enemy region for which it set out, the general guiding the army, or the doukator, must above all be quick and alert to send men out and secure information from prisoners through whom he will acquire accurate knowledge of the region.

It often happens that one or two days before the raid is launched, a body of reinforcements from somewhere else comes to the enemy, while the spies, deserters from the enemy, and captured prisoners coming from the place a week or three to four days previously are unaware of what has happened there one or two days before.

Nikephoros is describing that the light infantry and heavy cavalry (kataphraktoi) are converted into light cavalry (prokoursatores) to supplement the numbers of light cavalry during the march, bringing the numbers of light cavalry to between half and a third of the total force. While marching through enemy territory, the light cavalry split up into three groups, left, right and center. This formation frequently moves out to raid for provisions, but also prisoners, spies and anyone else that may provide useful information. It is also interesting that Nikephoros takes time to point out that new information is needs every few days, and after a few days a source can no longer be relied on. Based on this we can infer that the Byzantine Roman army on the march was mounting these types of maneuvers every few days, if not more often.

If contact with the enemy was made, the light cavalry would harass and bombard the enemy viscously while the kataphraktoi were readied. If the enemy were turned to flight, they were pursued in good order. If the enemy was steadfast, the light cavalry would pull back, hopefully enticing the enemy force to pursue into an ambush or a kataphraktoi charge. The feigned retreat being one of the Byzantine Roman's favorite tactics of this time period. If the enemy remained in good order and with great numbers the light cavalry could pull back with haste, notifying the rest of the force and organize a general withdrawal until circumstances were more favorable.

Overall, we can see two primary points, first, is that a good commander would only engage an enemy force if the conditions were favorable to him. It didn't really matter how many levies to enemy could field in their entirety, with superior intelligence and mobility, the Byzantine Roman army could take advantage of most situation, and refuse battle if no such opportunity was available. The second point is that, given the right circumstances and good generalship, a much smaller force had nothing to fear from a much larger one.

Sources

Praecepta militaria Nikephoros II Phokas

Taktika Nikephoros Ouranos

Strategikon Maurice Tiberius

edit for formatting

Like all "medieval" questions, the question is so broad as to be almost impossible to answer with any one single answer. The medieval period covers centuries and many different governing styles, cultures and political arrangements, and it's impossible to generalize. Your question of vassals could be construed to apply to England and France in the early middle ages, but not to Italy, where professional soldiers were far more common. But then, even during the 100 years war, there was frequent use of mercenaries and "called up" armies that were more than mere amateurs.

However, we can point to some specific examples, and those might be helpful.

Here is an article that discusses military espionage from the Wars of the Roses to the Reformation which is about the period you're interested in. It cites a large number of secondary sources and original documents, many of which would require language skills to access. As you might expect, it focuses primarily on england, but also has some references to contemporary scotland, france and italian city states.

It recites that the Ambassador were a primary source of political and military intelligence, and they were often the center of intelligence networks. We know of these, not in the least, because of financial records documenting monetary payments for intelligence purposes. For example, the English deputy at Calais had a budget of 104 pounds per year to employ spies, which was not an insubstantial sum.

Merchants were a major source of intelligence, and it was extremely common for government officials (in England, Italy and Elsewhere) to question merchants that had recently come from traveling in enemy held lands.

It also recounts that when war was threatened between Scotland and England, both Henry IV and Edward VII used a combination of disaffected scots, picard and various merchants, who would gather intelligence and report back to their respective kings. We have records of this through various crown payments to individuals and notations in this regard. For example, in 1839-1840 Richard Barowe of Holy Island was paid 28 pounds 6p for "certain matters touching the scottish war," and a Merchant, John Papdye, was paid 40 pounds.

Campaigning armies also frequently had spies, both near and far, to report on the disposition of enemy forces. In 1451 a ship containing a french speaking Scott, William Moneypenny, was wrecked on English Shores, and english authorities searched the ship for letters, which were found and immidiately sent to government authorities, and Moneypenny himself was jailed and held for ransom.

It also reflects the use of Espionage during the hundred years war. in 1454 the Earl of Arundel was allowed 1200 livres per year for spies, and 54,000 livres per year for "journeys, embassies and messangers" which would also have performed an intelligence function. Personal servants of a king or lord were also commonly used for intelligence gathering, as they could travel semi-anonymously.

One of the recurring points is that the 100 years war led to the development of the english state, and that intelligence gathering and a system of informers came along with this new state. Upon Henry VII's death, Thomas More remarked "'No longer does fear hiss whispered secrets in one's ear, for no one has secrets either to keep or to whisper. Now it is a delight to ignore informers. Only ex-informers fear informers now."

As a follow up, what would happen to a vassal and his men had he refused a call to war? What would happen to him and his forces during and after the war?

I believe I can get an accurate assessment for medieval kings in Western Europe during the early fourteenth century into the middle of the fifteenth. My sources for this will include Kelly DeVries's 'Infantry Warfare in the Early 14th Century', Clifford J.Rogers's various essays including 'The Edwardian Revolution in Military Affairs', Andrew Ayton's 'Knights and Warhorses', and Peter Coss's 'The Knight in Medieval England 1000AD-1500AD'.

To answer your question, I would say that the traditional methods of 'espionage' would be employed to some degree. Ambassadors were not quite codified, but representatives of opposing courts would likely gather information on the numbers and powers of the 'loyalist' magnates that would make up the majority of a king's armies. There's even some evidence to suggest that the retinues of magnates in this period were the 'core' of an army (infact, by the turn of the 15th century, they DEFINITELY were, see The Soldier In Later Medieval England, Adrain Bell, Anne Curry, Andy King, David Simpkin). So when, say, the retinue of the Duc d'Orleanais rocked up in Paris, an astute observer might ascertain it's size and strength, and link that to military strength.

However, the big kicker for this is that these are minor contributions compared to the constant stream of nobility moving inbetween the courts. Savoyards ended up in England, the French ended up in Prussia dining alongside Italian with the Teutonic Order. Nobility moved between courts constantly, with only landed nobility and their family and closest retainers staying anywhere for very long; the unlanded nobility could be found moving around a great deal. For example, while in exile, Henry Bolingbroke resided in France, and doubtless would have spoken to the military elite of France. A retainer who had served in France under the Duc d'Bourgogne could likely relate to the King of England that magnate's fighting capabilities. A prime example of this is Godfrey d'Harcourt, who worked alongside Edward III in his invasion of Normandy.

I hope this goes some way to answering your question!

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