How did territorial expansion lead to the fall of the roman empire?

The Roman Empire's fall was largely due to the army being spread too thin to defend the growing territory.  The army could not recruit fast enough to secure the huge increases in territory.  Also the relocation of men from their domestic lives within the city to the mobile army bases caused a lack of day to day workers within the city.  The economy suffered as result.

The answer is william henry harrison pursued and defeated british forces retreating into canada.