War has broken out in my world after 750 years of relative peace. After four years of back and forth fighting, the Marisari, now a nation weary of war, plan to win the final battle of the war by sending half of their army, a force of 1,000,000 elite warriors, to march on, then capture, the capital of the old federation. The only issue with this is that the only way to get to the capital is to go through 372 miles (100 yards) of untamed wilderness (grass). (I'm not sure if this is a short or long distance for an army mostly on foot to trek.). The capital is made aware of their plan and is told that they will arrive in roughly seven days if all goes well for them. This greatly distressed the capital, as they will need 12 days to prepare a defensive force and fortify the capital. So they send a force of 2,000 men on the backs of white and vinegar flies to delay the attackers. They are told upon going "Do whatever it takes, whatever it may be, to delay the usurpers "
My question is:
Is there anything that a force so insurmountably outnumbered could do to stall the force of 1,000,000 for five days?