The elder Filipescu died later that year, as Romania entered World War I an Entente ally; shortly after, an invasion by the Central Powers prompted the Ententist administration to withdraw into Moldavia. Grigore followed the Romanian administration and saw action on the front, advancing to the rank of Sub-lieutenant, while also joining the Labor Party, formed in 1917 by George Diamandy and other left-wing defectors from the PNL.