re-ally v.

word-forming element meaning "back, back from, back to the original place;" also "again, anew, once more," also conveying the notion of "undoing" or "backward," etc. (see sense evolution below), c. 1200, from Old French re- and directly from Latin re- an inseparable prefix meaning "again; back; anew, against."

Watkins (2000) describes this as a Latin combining form conceivably from Indo-European *wret-, metathetical variant of *wert- "to turn." De Vaan says the "only acceptable etymology" for it is a 2004 explanation which reconstructs a root in PIE *ure "back."

In earliest Latin the prefix became red- before vowels and h-, a form preserved in redact, redeem, redolent, redundant, redintegrate, and, in disguise, render (v.). In some English words from French and Italian re- appears as ra- and the  following consonant is often doubled (see rally (v.1)).

The many meanings in the notion of "back" give re- its broad sense-range: "a turning back; opposition; restoration to a former state;" "transition to an opposite state." From the extended senses in "again," re- becomes "repetition of an action," and in this sense it is extremely common as a formative element in English, applicable to any verb. OED writes that it is "impossible to attempt a complete record of all the forms resulting from its use," and adds that "The number of these is practically infinite ...."   

Often merely intensive, and in many of the older borrowings from French and Latin the precise sense of re- is forgotten, lost in secondary senses, or weakened beyond recognition, so that it has no apparent semantic content (receive, recommend, recover, reduce, recreate, refer, religion, remain, request, require). There seem to have been more such words in Middle English than after, e.g. recomfort (v.) "to comfort, console; encourage;" recourse (n.) "a process, way, course." Recover in Middle English also could mean "obtain, win" (happiness, a kingdom, etc.) with no notion of getting something back, also "gain the upper hand, overcome; arrive at;" also consider the legal sense of recovery as "obtain (property) by judgment or legal proceedings." 

And, due to sound changes and accent shifts, re- sometimes entirely loses its identity as a prefix (rebel, relic, remnant, restive, rest (n.2) "remainder," rally (v.1) "bring together"). In a few words it is reduced to r-, as in ransom (a doublet of redemption), rampart, etc.

It was used from Middle English in forming words from Germanic as well as Latin elements (rebuild, refill, reset, rewrite), and was used so even in Old French (regret, regard, reward, etc.).

Prefixed to a word beginning with e, re- is separated by a hyphen, as re-establish, re-estate, re-edify, etc. ; or else the second e has a dieresis over it: as, reëstablish, reëmbark, etc. The hyphen is also sometimes used to bring out emphatically the sense of repetition or iteration : as, sung and re-sung. The dieresis is not used over other vowels than e when re is prefixed : thus, reinforce, reunite, reabolish. [Century Dictionary, 1895]

late 13c., allien, "join in marriage" (transitive), from Old French alier "combine, unite," from a differentiated stem of aliier (from Latin alligare "bind to, tie to," from ad "to" (see ad-) + ligare "to bind, bind one thing to another, tie" (from PIE root *leig- "to tie, bind"). The meaning "form an alliance, join, associate" is late 14c. Related: allied; allying.

"bring together or into order again by urgent effort," c. 1600, from French rallier, from Old French ralier "reassemble, unite again," from re- "again" (see re-) + alier "unite" (see ally (v.)).

In Old French (and Italian), re- often appeared as ra- by confusion with the true ra- (from Latin re- + ad-), and the following consonant often was doubled; compare rabbet (a doublet of rebate), rappel (a doublet of repeal). But as ra- was not recognized in English as a prefix, words with ra- in Old French usually returned to re- in English; Rally and rabbet never were because the accent had receded. In later borrowings (rappel, rapprochement, etc.) the words tend to keep their French forms. 

Intransitive meaning "pull together hastily, recover order, revive, rouse" is from 1660s. Related: Rallied; rallying. Rallying-point "place at or about which persons come together for action" is by 1798. Rally round the flag (1862) is a line from popular American Civil War song "Battle Cry of Freedom."