To expand on the question above:
In 1601, France and Savoy formed a Treaty, which in practice disallowed the Spanish access they previously had through Savoy. How did this affect Spanish transport of supplies and men to the possessions passed to the Spanish from the succession of Burgundy? Could Spain still reliably get supplies to the Seventeen States, Franche Comte, etc., If so, how?
u/ByoByoxInCrox, great question.
The 1601 Treaty of Lyon did severely restrict Spanish ability to cross the Rhone west of the Swiss cantons, leaving only the narrow valley -- the Val de Chezery -- and one single bridge the Pont de Gresin.
While it was clear that Spain's dominance was waning, but it wasn't yet the point of impossibility. Provably, months after the Treaty of Lyon was signed, Ambrogio Spinola was able to bring a tercio of about 6,000 foot and 1,000 mounted through this narrow pathway with Savoy support.
What made the Spanish Road untenable was the shift of Savoy away from Spanish orbit into outright alliance with France. This shift happened gradually. In the 1500s, the Dukes of Savoy sided with Spain to protect themselves against Henry IV's ambition against them. After 1601, the Duke of Savoy still felt he needed Spain patronage to secure his flanks against France. Even after the Treaty of Lyon, Savoy invited Spanish tercios to garrison some forts in the Val de Chezery. This arrangement lasted about a decade, until Savoy started to look toward Spanish Milan with ambition.
However, even as the paths west of the Swiss cantons became unusable, Spain had had designs to open roads east of the Swiss cantons, including through Valtellina where Catholic Swiss cantons tended to be more friendly, to continue to Alsace. To quote Cardinal Granvelle, a person familiar with these routes, "[the route through the Catholic cantons] are shorter, but is neither as commodious nor as safe as the Canton route." Thus, a path across the Rhine was opened.
Of course, success is never final, but that's for another post.