He spent most of his adult life as a military officer, first in the British army, then he was commander of the US Continental army.
yes
In early July of 1862, US President Lincoln summoned General Henry W. Halleck to come to Washington DC. Lincoln had already decided to give Halleck command of all Federal armies, which of course included General Geprge B. McClellan's Army of the Potomac. Halleck's fist assignment was to see if McClellan was willing to retry his assault on the Confederate capital of Richmond. Lincoln had focused on the fall of Richmond since the beginning of the war. Halleck was informed that McClellan would have to use the troops he already had. If McClellan refused, Halleck was to relieve him and withdraw the army back to Washington DC and an entirely new campaign could be formulated.
George and Lennie hide in an irrigation ditch to escape from a confrontation that arose from Lennie's unintentional actions. They hope to avoid trouble and to find a safe place to hide until they can figure out their next steps. This allows them to temporarily evade any potential consequences of Lennie's behavior.