The apogee of his political trajectory fell on early period of the Civil War, when he was president of the Alavese Diputación Provincial (1936–1938) and leader of Falange Española Tradicionalista in Álava (1937).

At times he is also acknowledged as a polemist of Sabino Arana (mid-1890s), who inadvertently helped the latter to refine his theory of Basque nationalism, and as a point of reference for the Carlist position towards fueros.

According to some sources he spent his early years in Elciego, with the maternal family, though at some stage he joined his father in Zaragoza; it is unclear whether he received secondary education in the Aragonese capital.

[22] Already during his university spell he demonstrated interest in letters, and especially in journalism; following graduation in 1894[23] he did not launch a career in law, but moved to Bilbao to assume the post of editor-in-chief of the newly set up periodical of the Carlist youth, Chapel-Zuri.

His grandfather was a vehement supporter of separate Basque establishments, and in the mid-19th century in open letters he protested against "anexion de este país á una nación extranjera",[39] which might or might have not translated into Carlism.

[43] His first identified contribution to the national Carlist mouthpiece, El Correo Español, is dated 1893; at the time in the official party structures he held the post of vicesecretario of Junta Provincial.

[54] For some time he appeared briefly related to an ephemerical Carlist periodical from Vitoria, El Alavés,[55] and to a short-lived weekly from Logroño, La Lealtad Riojana.

[56] His best-known articles of the time are these published in Chapel-Zuri and El Basco; they are related to increasingly bitter polemics with ideologue of the nascent Basque nationalism, Sabino Arana.

Under Echave-Sustaeta's guidance El Pensamiento Navarro emerged as a vehemently Carlist newspaper; it was intended for a possibly broad audience and prioritized news and short articles over doctrinal and ideological dissertations.

[67] El Pensamiento stood out due to its combative and intransigent style; in 1900 it cost Echave-Sustaeta detention, related also to general anxiety about another Carlist insurgency forthcoming,[68] and in 1901 a hefty fine, reduced in course of the appeal process.

[73] Echave-Sustaeta's term at El Pensamiento Navarro coincided with the period of Carlist political domination in the region; the party controlled 5-6 out of 7 Cortes mandates, allocated to Navarre, but none of the dedicated works discusses the newspaper's role.

[74] Echave-Sustaeta later recollected that though he was assured there were prestigious individuals ready to collaborate, in fact he struggled to ensure reasonable content and remained dramatically short of good contributors.

[75] Similarly, much later he admitted that technologically the daily remained somewhat antiquated and behind leading newspapers of the time;[76] he quoted limited circulation and constant financial constraints as the key reason.

[77] Another problem was increasing fragmentation of Carlism of the 1910s, related to party policy versus peripheral nationalisms, conflict between the Mellistas and the claimant, or question of political alliances.

[78] In the late 1890s Echave-Sustaeta entered editorial board of Biblioteca Popular Carlista, a series of booklets and pamphlets issued by a Catalan Traditionalist publishing house.

[89] He proved a rather restless and combative member; in 1906 he protested a motion to commemorate liberal politicians Castelar, Pi y Margall and Sagasta, refused to budge, was fined, paid at the spot, left the room and took other Carlists councilors with him, breaking up the entire session.

[102] It was prologued by Juan Vázquez de Mella, at the time the key Carlist theorist, Cortes deputy and as great speaker sort of celebrity, which positioned the book as a quasi-official party voice on the issue.

The volume was intended to demonstrate that throughout the last 100 years Traditionalism was the only current which genuinely incorporated heterogeneity of local regimes into its doctrine and that Carlism was the only party which systematically strove to defend them against centralisation of liberalism-driven Madrid governments.

[112] Some press notes from the early 1920s confirm that he was active there, as he was mentioned in related societé[113] or political columns,[114] and was referred to as "el abogado en Estella".

At the turn of the decades Echave-Sustaeta found himself in the shadow of his younger brother Antonio,[117] who emerged as key figure within the Alavese diputación provincial and in the hostile liberal press was ridiculed as a politician who believed that "he was the boss and the master of the province".

[118] Eustaquio remained moderately engaged in Carlism, e.g. in 1918 he supported the party candidate Gervasio de Artiñano y Galdácano, who was running for the Cortes from the Laguardia district.

[119] At the time the movement was increasingly paralyzed by the conflict between the key theorist Vázquez de Mella and the claimant Don Jaime, yet in historiography Echave-Sustaeta is not listed as committed to any of the sides.

[120] When in 1919 the dispute erupted into full-blown confrontation he stayed loyal to his king; during the Cortes electoral campaign of 1920 he supported the Jaimista candidate Esteban Bilbao, who was successfully running from the Estella district.

[139] When dictatorship was crumbling Echave-Sustaeta politically remained in the shadow of his brother; a 1930 manifesto, issued by Carlist executive, in name of the Alavese branch was signed by Antonio.

Within structures of the newly created state party, Falange Española Tradicionalista, on May 1 he was nominated one of 9 Delegados Provinciales (provincial leaders) who came with the Carlist background (against 22 Falangist old-shirts).

[182] In August 1937 the post of civil governor was ceded by colonel Fernández Ichaso to Eladio Esparza; earlier marginally related to Carlism, at the time he already represented "a new vision".

[185] In early October 1937 Echave-Sustaeta was replaced as FET Delegado Provincial by José María de Elizgarate; an official statement read that he had been asking for release for some time.

[190] It did not last for long; though as late as in January 1938 Echave-Sustaeta was engaged in maneuvers intended to save local Gipuzkoan prerogatives,[191] in March 1938 he quoted health reasons and resigned from the self-government;[192] the moment marked the end of "diputacion oriolista".

[193] Until the early 1940s it was his brother Antonio who served as secretario of the Diputación, which kept generating constant backlash on part of the Falangist old-shirts;[194] it eventually produced his dismissal with charges of financial embezzlement.

[198] As a septuagenarian, in the press of the 1940s he was noted only when donating money to patriotic causes, e.g. in support of División Azul,[199] because of family reasons[200] or as "El Jefe" of Cooperativa Vinícola de Elciego, a local wine-growing cooperative.