vre , ▁Duke ▁John ▁III ' s ▁younger ▁full ▁brother , ▁and ▁thus ▁John ▁III ' s ▁full ▁nie ce . ▁ ▁John ▁of ▁Mont fort ▁however ▁invoked ▁both ▁the ▁principle ▁of ▁Sal ic ▁law ▁( allow ing ▁only ▁males ▁to ▁succeed ) ▁and ▁the ▁principle ▁of ▁proxim ity ▁of ▁blood ▁( as ▁used ▁earlier , ▁e . g . ▁by ▁John ▁of ▁England ▁against ▁Arthur ▁I ▁of ▁Brit t any ), ▁having ▁himself ▁proc laimed ▁Duke . ▁This ▁led ▁to ▁the ▁Bre ton ▁War ▁of ▁Su cc ession , ▁a ▁part ▁of ▁the ▁H undred ▁Years ' ▁War . ▁His ▁patron ▁in ▁this ▁quest ▁was ▁king ▁Edward ▁III ▁of ▁England . ▁The ▁riv als , ▁Duch ess ▁Jo anna ▁and ▁her ▁husband ▁Charles ▁of ▁Blo is ▁were ▁supported ▁by ▁the ▁Val ois ▁k ings ▁of ▁France . ▁John ▁of ▁Mont fort ▁died ▁without ▁accomplish ing ▁his ▁objective ▁of ▁becoming ▁sole ▁r uler ▁of ▁Brit t any , ▁but ▁his ▁wife ▁Jo anna ▁of ▁Fland ers ▁continued ▁the ▁fight ▁in ▁the ▁name ▁of ▁their ▁son ▁John ▁IV , ▁Duke ▁of ▁Brit t any ▁( 1 3 3 9 / 4 0 – 9 9 ) ▁who ▁ultimately ▁got ▁the ▁upper ▁hand ▁in ▁the ▁str ife . ▁ ▁In ▁the ▁midst ▁of ▁the ▁conflict , ▁in ▁ 1 3 5 2 , ▁the ▁Est ates ▁of ▁Brit t any ▁( Ét ats ▁de ▁Bret agne ) ▁were ▁established . ▁They ▁would ▁develop ▁into ▁the ▁Duch y ' s ▁par lement . ▁ ▁R uling