The last part of Burgoyne's proposal, the advance by Howe up the Hudson from New York City, proved to be the most controversial part of the campaign. Germain approved Burgoyne's plan after having received Howe's letter detailing his proposed offensive against Philadelphia. Whether Germain told Burgoyne, who was still in London at that time, about Howe's revised plans is unclear: while some sources claim he did, others state that Burgoyne was not notified of the changes until the campaign was well underway. Historian Robert Ketchum believes that Burgoyne would probably have been aware of the problems that lay ahead had he been notified of the Philadelphia plan.