Francis will be seen on the battlefield in "Reign" Episode 20, which is titled "Higher Ground." The preview clip of the episode suggests that Francis and his army are in a difficult position and on the verge of losing the war.
In the previous episode, "Toy Soldiers," King Henry opened the door for this war to get back Calais from England, which according to him would help France claim England. He sent out a team of France's army, without informing anyone, leaving Francis with no option but to consider the interest of France over Scotland, which is in dire need of military support.
Francis enlisted the help of Mary's uncle, Duke of Guise, to win his battle. According to the original plan, Duke of Guise and his army were supposed to march towards Scotland to help Marie de Guise.
Mary had invited her uncle to the castle so that he and his army could help Scotland and her mother -- who is surrounded by an army of Protestants. Francis reneged on his promise to help Scotland and lead Duke of Guise's army to save her mother. He wanted to first secure his country's interests and then proceed to protect Scotland.
"Toy Soldiers" ended with Mary saying that she would not let Francis put at risk Scotland, again. The synopsis of the episode suggests that Mary will be hiring a mercenary to save her country.
Meanwhile, Leith returns in this episode and becomes an ally of Francis to win the war against England. Greer and Lord Castleroy's relationship is strengthening and if Leith returns as part of Francis's victorious army, there are bound to be hiccups in the new relationship.
Also, Julien and Lola's relationship will solidify, though Lola remains suspicious about Julien's real intention and fears that he will abandon her.
Official Synopsis of "Reign" Episode 21, "Higher Ground": "Mary hires a mercenary to help her save Scotland. Meanwhile, Francis prepares France to wage war on England, and Leith becomes his ally in this endeavour; and Lola and Julien's bond deepens, but she fears he will betray her."