Here:

At the end of 1919 the Ukrainian Army was forced by the superior Bolshevik and anti-Bolshevik Russian forces to retreat westward where it encountered the Polish Army. Petliura decided it was impossible under these circumstances to continue orthodox warfare without external aid. He felt, however, that the struggle against the invading Russian forces should continue in the form of guerrilla warfare. Part of the army, about fourteen thousand men, was reorganized and, on December 7, 1919, under General Mykhailo Omelianovych-Pavlenko, moved far to the rear of the Bolshevik and Denikin forces to fight them and to support the Ukrainian partisans. This “Winter Campaign” continued through the winter of 1919–20.[15]

Petliura realized that Ukraine could not survive in her struggle against the Russian Reds and Whites without assistance from the Entente. Therefore, he tried to come to terms with the Polish government, which, in his judgment, was the bridge to the Entente. At first reluctant, Poland came awake when the erosion of the Ukrainian position threatened to remove the barrier that had protected Poland from the Bolshevik threat since Germany’s defeat. Thus, once the Polish Army attained the desired frontiers, the Zbruch River and western Volyn up to the Styr River, at the expense of Ukraine, the Poles agreed to negotiate. The Directory was obliged to issue a declaration on December 2, 1919, without the consent of its Galician members, accepting this line as the Polish-Ukrainian frontier.

After prolonged negotiations the two governments concluded a treaty, consisting of a political agreement and a military convention. Both governments expressed their profound conviction “that each people possesses the natural right to self-determination and to define its relations with neighboring peoples, and is equally desirous of establishing a basis for concordant and friendly coexistence for the welfare and development of both peoples.”[16] Although the treaty had many weaknesses and was sharply criticized by many on both sides, and in addition it did not achieve the Directory’s hope of winning French or British support, both the Polish and Ukrainian armies, including the participants in the winter campaign, joined in fighting the Red Army. On May 8, 1920, they liberated Kyiv. The Bolshevik counteroffensive, however, forced them to retreat deep into Poland where the Bolsheviks were finally defeated. The Soviet Russian government proposed an armistice and a preliminary peace that the Polish government accepted without consulting the Ukrainian government.

This is from an explicitly pro-anarchist, pro-Makhno source, so this isn’t really controversial. The justification tends to be that Makhnovschina was playing both sides, in order to best secure its position in a chaotic landscape. I didn’t dare being a pro-soviet source as it becomes even more damning, but at the same time would be less likely to be taken seriously.

The Makhnovists didn’t hide their animosity towards the Bolsheviks either, so this shouldn’t be surprising that they would fight:

Military hostilities between the Makhnovist revolutionary insurgents and the Red Army have ceased. Misunderstandings, vagueness and inaccuracies have grown up around this truce: it is said that Makhno has repented of his anti-Bolshevik acts, that he has recognized the soviet authorities, etc. How are we to understand, what construction are we to place upon this peace agreement?

What is very clear already is that no intercourse of ideas, and no collaboration with the soviet authorities and no formal recognition of these has been or can be possible. We have always been irreconcilable enemies, at the level of ideas, of the party of the Bolshevik-communists.

We have never acknowledged any authorities and in the present instance we cannot acknowledge the soviet authorities. So again we remind and yet again we emphasize that, whether deliberately or through misapprehension, there must be no confusion of military intercourse in the wake of the danger threatening the revolution with any crossing-over, ‘fusion’ or recognition of the soviet authorities, which cannot have been and cannot ever be the case.

[Source: Nestor Makhno: Anarchy’s Cossack by Skirda and Sharkey, pp. 200-201]

The Russian Civil War created strange bedfellows. The idea that Makhnovschina tried to play both sides for its own interests is an extremely charitable reading of how they participated in the civil war. If the Red Army was truly the threat to Makhnovschina the Makhnovists said it was, then it makes perfect sense why they would temporarily form alliances against the Red Army if only to try to stay afloat themselves. And, for what it’s worth, they sided with the Red Army for the majority of the war.

Not everything is an evil communist conspiracy, sometimes Civil Wars are messy and result in strange bedfellows.