Simplify the following text for a Grade 5 native English-speaking person.
Original: While he was already involved in the Gaelic Revival , de Valera 's involvement in the political revolution began on 25 November 1913 when he joined the Irish Volunteers formed to oppose the Ulster Volunteers and ensure the enactment of the Irish Parliamentary Party 's Third Home Rule Act won by its leader John Redmond .
Simplified: On 25 November 1913 when he joined the Irish Volunteers .