From the OED (Oxford English Dictionary)
Definition of commandeer
verb
[with object]
officially take possession or control of (something), especially for military purposes: a nearby house had been commandeered by the army
take possession of (something) by force: the truck was commandeered by a mob
[with object and infinitive] enlist (someone) to help in a task: he commandeered the men to find a table
Origin:
early 19th century: from Afrikaans kommandeer, from Dutch commanderen, from French commander 'to command' (see command)
Note the very important first word " officially"
Also from the OED
Definition of hijack
verb
[with object]
illegally seize (an aircraft, ship, or vehicle) while in transit and force it to go to a different destination or use it for one’s own purposes: a man armed with grenades hijacked the jet yesterday
steal (goods) by seizing them in transit: the UN convoys have been tamely allowing gunmen to hijack relief supplies
take over (something) and use it for a different purpose: he argues that pressure groups have hijacked the environmental debate
noun
an incident or act of hijacking: [as modifier]: an unsuccessful hijack attempt
Derivatives
hijacker
noun
Origin:
1920s (originally US): of unknown origin
Also note the very important first word "illegally".
QED