user11674: According to the historical story before the day of enlightenment, Mara apparently lured the Buddha with various worldy pleasures. Now, I am not arguing whether Mara really existed or is just Buddha's mind (which would be rather my interpretation). But The Buddha should at that time be a non-returner and he must have abandoned the fetter of sensuous desires. He should've chilled in the 4th Jhana not giving a damn!
So my question is: Why was it a struggle for the Buddha if the conditioned fetter of sensuous desire was already uprooted as an anagami?

user43: In my understanding, Buddha did not progress through the four stages of enlightenment he later prescribed for his students.
This is because, for Buddha students - you start with theoretical understanding (stream entry), and then you practice according to instructions until you achieve it (arahant-hood).
In Buddha's case, because he is Self-Awakened, he starts with intuitive practice and then achieves understanding (Bodhi). 
So in Buddha's case he does not proceed gradually, phase by phase, like assembling the puzzle by looking at the complete picture. Instead, it's like solving a puzzle randomly - by the time he sat under the Bodhi Tree, he had assembled most of the pieces but still did not know what was shown on the picture.
This is why in Buddha's case he left all interpretative possibilities open until the last moment, which means he still had doubts until the very last moment. Those doubts are represented in the anthropomorphic personification known as "the Mara".
In case of Buddha's students, the interpretative framework is locked in place from the time of stream-entry, so doubts are eliminated at that phase.

user254: For reference, according to this account1, it was Mara's choice to assail the Buddha -- Mara is presented as another/separate being, an agent, a divinity (and not as a metaphor for unenlightened aspects of the future Buddha's mind)2:

At this point the god Mâra, exclaiming, "Prince Siddhattha is desirous of passing beyond my control, but I will never allow it!" went and announced the news to his army, and sounding the Mâra war-cry, drew out for battle.

And in that "struggle" the Buddha used (not the 4th Jhana but) his recollection of (or his accomplishment of, his perfection of) the Ten Perfections, five great donations, struggle for the welfare of the world, for example:

My mother and father are not here, nor my brother, nor any other relative. But I have these Ten Perfections, like old retainers long cherished at my board. It therefore behooves me to make the Ten Perfections my shield and my sword ...


If you want to interpret it metaphorically, perhaps see it as his "Am I worthy?" struggle with "conceit" (which is one of the last fetters) -- e.g. because "conceit" includes "comparison", and this passage includes explicit comparison between the Buddha and Mara.
Or see as an affirmation or explanation for us, to help explain how we should view him as worthy.

I think the recollection of perfections may also echo some of the knowledge associated with final enlightenment:

The Arahant SN 22.110
And when, monks, a monk, having seen as they really are the arising and the passing away, the attractiveness and the danger, and the deliverance from the five groups of clinging, is released without clinging, he, monks, is called a monk in whom the cankers are destroyed, who has lived the life to perfection, done what had to be done, put down the burden, gained the highest goal, worn through the fetters of rebirth, and is liberated by perfect insight.
(The whole phrase is a standard description of the Arahant found at many places in the Canon)

Speaking of "perfections" in general, IMO the English word "perfect" has at least three related meanings, i.e., 1) very good (high quality); 2) complete (finished, past tense); 3) whole.

1The Attainment of Buddhaship -- Translated from the Introduction to the Jâtaka -- referenced from this answer
2There are other records of discourse between the Buddha and Mara, which happen after the Buddha's awakening -- for example even in the Maha-parinibbana Sutta:

And when the Venerable Ananda had gone away, Mara, the Evil One, approached the Blessed One. And standing at one side he spoke to the Blessed One, saying: "Now, O Lord, let the Blessed One come to his final passing away; let the Happy One utterly pass away! The time has come for the Parinibbana of the Lord.


11674: <p>According to the historical story before the day of enlightenment, Mara apparently lured the Buddha with various worldy pleasures. Now, I am not arguing whether Mara really existed or is just Buddha's mind (which would be rather my interpretation). But The Buddha should at that time be a non-returner and he must have abandoned the fetter of sensuous desires. He should've chilled in the 4th Jhana not giving a damn!</p>

<p>So my question is: Why was it a struggle for the Buddha if the conditioned fetter of sensuous desire was already uprooted as an anagami?</p>
 43: <p>In my understanding, Buddha did not progress through the four stages of enlightenment he later prescribed for his students.</p>

<p>This is because, for Buddha students - you start with theoretical understanding (stream entry), and then you practice according to instructions until you achieve it (arahant-hood).</p>

<p>In Buddha's case, because he is Self-Awakened, he starts with intuitive practice and then achieves understanding (Bodhi). </p>

<p>So in Buddha's case he does not proceed gradually, phase by phase, like assembling the puzzle by looking at the complete picture. Instead, it's like solving a puzzle randomly - by the time he sat under the Bodhi Tree, he had assembled most of the pieces but still did not know what was shown on the picture.</p>

<p>This is why in Buddha's case he left all interpretative possibilities open until the last moment, which means he still had doubts until the very last moment. Those doubts are represented in the anthropomorphic personification known as "the Mara".</p>

<p>In case of Buddha's students, the interpretative framework is locked in place from the time of stream-entry, so doubts are eliminated at that phase.</p>
 254: <p>For reference, according to <a href="http://www.sacred-texts.com/bud/bits/bits008.htm" rel="nofollow noreferrer">this account</a><sup>1</sup>, it was Mara's choice to assail the Buddha -- Mara is presented as another/separate being, an agent, a divinity (and not as a metaphor for unenlightened aspects of the future Buddha's mind)<sup>2</sup>:</p>
<blockquote>
<p>At this point the god Mâra, exclaiming, &quot;Prince Siddhattha is desirous of passing beyond my control, but I will never allow it!&quot; went and announced the news to his army, and sounding the Mâra war-cry, drew out for battle.</p>
</blockquote>
<p>And in that &quot;struggle&quot; the Buddha used (not the 4th Jhana but) his recollection of (or his accomplishment of, his perfection of) the Ten Perfections, five great donations, struggle for the welfare of the world, for example:</p>
<blockquote>
<p>My mother and father are not here, nor my brother, nor any other relative. But I have these Ten Perfections, like old retainers long cherished at my board. It therefore behooves me to make the Ten Perfections my shield and my sword ...</p>
</blockquote>
<hr />
<p>If you want to interpret it metaphorically, perhaps see it as <strong>his</strong> &quot;Am I worthy?&quot; struggle with &quot;conceit&quot; (which is one of the last fetters) -- e.g. because &quot;conceit&quot; includes &quot;comparison&quot;, and this passage includes explicit comparison between the Buddha and Mara.</p>
<p>Or see as an affirmation or explanation for <strong>us</strong>, to help explain how we should view him as worthy.</p>
<hr />
<p>I think the recollection of perfections may also echo some of the knowledge associated with final enlightenment:</p>
<blockquote>
<p><a href="https://www.accesstoinsight.org/tipitaka/sn/sn22/sn22.110.wlsh.html" rel="nofollow noreferrer">The Arahant SN 22.110</a></p>
<p>And when, monks, a monk, having seen as they really are the arising and the passing away, the attractiveness and the danger, and the deliverance from the five groups of clinging, is released without clinging, he, monks, is called a monk in whom the cankers are destroyed, who has <strong>lived the life to perfection, done what had to be done</strong>, put down the burden, gained the highest goal, worn through the fetters of rebirth, and is liberated by perfect insight.</p>
<p>(The whole phrase is a standard description of the Arahant found at many places in the Canon)</p>
</blockquote>
<p>Speaking of &quot;perfections&quot; in general, IMO the English word &quot;perfect&quot; has at least three related meanings, i.e., 1) very good (high quality); 2) complete (finished, past tense); 3) whole.</p>
<hr />
<p><sup>1</sup><em>The Attainment of Buddhaship</em> -- Translated from the Introduction to the Jâtaka -- referenced from <a href="https://buddhism.stackexchange.com/a/14366/254">this answer</a></p>
<p><sup>2</sup>There are other records of discourse between the Buddha and Mara, which happen after the Buddha's awakening -- for example even in the <a href="https://www.accesstoinsight.org/tipitaka/dn/dn.16.1-6.vaji.html" rel="nofollow noreferrer">Maha-parinibbana Sutta</a>:</p>
<blockquote>
<p>And when the Venerable Ananda had gone away, Mara, the Evil One, approached the Blessed One. And standing at one side he spoke to the Blessed One, saying: &quot;Now, O Lord, let the Blessed One come to his final passing away; let the Happy One utterly pass away! The time has come for the Parinibbana of the Lord.</p>
</blockquote>
