【Strengthen】 , 【invigorate】 ,  【fortify】 ,  【energize】 ,  【reinforce】  can mean to make strong or stronger.

【Strengthen】 is the most general term, applicable not only to persons or their physical, mental, or moral powers but also to things material or immaterial (as a structure, a system, an aggregation, or an influence); the word can connote increase either in force, energy, vigor, and power of resistance, or in intensity, authority, or effectiveness.

【Invigorate】 is commonly used in reference to living things or of some power or activity of a living thing and is sometimes extended to things that have powers suggestive of life; it implies an increase in vigor or vitality or active or effective strength or force.

【Fortify】 , which primarily means to 【strengthen】 a town or city by defensive structures, in a more general sense means to 【strengthen】 against attack or stress of any sort.

【Energize】 implies a strengthening in the form of supplying the active power for working or for being ready for work and, therefore, is used of whatever can be roused into strong activity or infused with a desire to act.

【Reinforce】 implies the making of what is weak stronger or of what is strong still stronger by or as if by an addition that stiffens and supports, and thereby adds effectiveness, powers of resistance, cogency, or durability.