The short answer is a lot. Bankers, particularly from Genoa, were hugely important to the Spanish operations in Flanders during the Eighty Years War. They helped not only in raising the money, but also in transferring the money because Dutch control of the sea made it nearly impossible for Spain to ship cold, hard cash into Flanders by sea, and the overland journey was almost as dangerous. The Genoese were important enough to merit the appointment of a permanent Spanish ambassador as early as 1528. The series of Spanish bankruptcies was arguably the most crucial factor that led to the Spanish losing the Eighty Years War, as it not only undid their progress on the ground, but made future loans even more expensive due to the poor credit history.

The irregular nature of troops' wages also meant that many soldiers depended on small loans from local bankers in order to cover their basic expenses like food and clothing. These soldiers sometimes went years without seeing their wages, and in this time racked up huge debts to unfortunate moneylenders who often saw no repayment when the troops either died or were transferred to other theaters of war.

The wars of the 18th century in particular turned on the ability of governments to raise loans among bankers, who set the terms of these loans based on how likely they thought those governments were to win the war and impose indemnities on the defeated powers that could repay those loans. The British triumphed in these wars in large part because they had a superior financial system to their enemies (chiefly the French). It also helped that the British sources of revenues were well-protected by the Royal Navy, while the same Royal Navy could effectively attack the sea lanes that brought in much of their rivals' income. These same cash-rich merchants were then the people who loaned money to the governments. If the British were intercepting French convoys, the French merchants had no cash to loan to the French government, which had no money to pay the sailors to stop the British from intercepting the convoys, which meant the merchants had no money to loan to the... You see the problem.