As I understand, military alliances were formed through marriage, and would at times support each other in war. When a lord would raise his levies and begin the march, how did they go about passing through neutral territory?
How did they manage supplies in these neutral lands? Did they forage off the land? And if so, did this upset the owners of the neutral territory they were passing through?
Are there any examples of an army displeasing local rulers by taking supplies from neutral territories?
Things could sometimes get very hairy. For instance the march of the first crusaders through Hungary in 1096 was fraught with difficulties. The forerunners of the crusade, called the people's crusade, arrived first and some of the members of the contingent led by Peter the Hermit got into a fight over the price of a pair of shoes in Zemun, modern Serbia. This led to a riot and the massacre of 4000 Hungarians. Another column pillaged the region of Nytria until they were routed by the locals and another pillaged the region around Mosonmagyaróvár until king Coloman arrived and routed them as well. Albert of Aix, author of the history of the journey to Jerusalem, notes that "[They] were even granted a licence to buy and sell necessary supplies, and peace was proclaimed on both sides according to [Coloman's] instructions, lest a dispute might arise from such a large army. But when they were delayed there for some days, they began to wander, and the Bavarians and Swabians, a bold race, and the rest of the soldiers foolishly drank too much; they violated the proclaimed peace, little by little stealing wine, barley, and other necessities from the Hungarians, finally seizing sheep and cattle in the fields and killing them". Yet another section of the peoples crusade led by Emicho were denied entrance to Hungary but attacked anyway, Coloman mustered him army and routed him as well.
When the actual crusaders arrived Coloman negotiated a hostage taking for good behavior and released them when they passed beyond his borders. The crusaders (mostly the peoples crusaders) continued to make trouble, pillaging the countryside, as they passed through Byzantium even though emperor Alexios took great pains to provide for them with special markets at discounted prices. A notable example that always makes me laugh is that when they made it to Constantinople they began to burn the suburbs and steal the lead off church roofs and then sell it back to the extremely annoyed populace. Alexios was forced to send out some crack cavalry under his son in law Nikephorus Byrennios and defend the walls of Constantinople from attacks by Godfrey of Boullion. However, I get the feeling that they were exceptionally rowdy even by medieval standards.
Christopher Tyreman: God's War: A New History of the Crusades
Steven Runciman: a History of the Crusades
Lars Brownworth: Twelve Byzantine Emperors